PROPOSAL: 01193     PI: Joern Wilms

The hard state of galactic black holes: nustar observations of GRS 1758-258

   We propose a 20 ks observation of GRS 1758-258, the first broad band observation of this canonical, persistent hard state black hole candidate with an imaging instrument. Our aim is to study the broad-band X-ray spectrum, to measure the broad-band short-term X-ray spectral variability, and to perform the first sensitive search for a relativistically broadened Fe Kalpha line.

PROPOSAL: 03054     PI: Raffaella Margutti

DETAILED MAPPING OF EXTREME MASS LOSS WITH COORDINATED NUSTAR-XMM OBSERVATIONS

    Contrary to expectations from current stellar evolutionary models, recent observations uncovered the ejection of shells of material by massive stars in the years before the supernova (SN) explosion. The physical mechanism behind the impulsive mass ejection synchronized with the stellar core-collapse is unclear. Here we propose a coordinated NuSTAR-XMM effort to map the evolution of the broad-band X-ray spectrum of 1 nearby (d<50 Mpc) strongly interacting SN and enable progress. Our program has the immediate goal to characterize the medium around strongly interacting SN, which originate from stellar progenitors with the most extreme mass loss before explosion.

PROPOSAL: 03059     PI: Kristin Madsen

CATCHING THE FLARING JET OF 3C 273

    The gamma-ray flaring of the jet in 3C273 is a rare event and seen just twice over the last 5 years. Only once before has a hard X-ray instrument been pointed at 3C273 during such a confirmed event, and we request 75ks to observe the impact of the bright jet state on the X-ray band. NuSTAR is the only observatory currently capable of probing the hard X-ray turnover with sufficient sensitivity during such a rare event, and the observation will provide unprecedented details on this complex transition region between AGN and jet, and perhaps once and for all solve the mystery of whether the curvature of the hard X-ray spectrum comes primarily from the accretion flow of the AGN component, or from a SSC+ERC jet.

PROPOSAL: 03080     PI: Javier Garcia

THE EVOLUTION OF THE ACCRETION DISK IN GX 339-4

    X-ray reflection spectroscopy is one of the most effective means of probing the strong-field region near an event horizon. Modeling a reflection spectrum, one can constrain the spin of a black hole, the degree of truncation of the inner disk, the inclination of the system, and other key parameters. We request 80 ks of NuSTAR time to trigger four observations of the black hole binary GX 339-4 during the rise of its next outburst. Our principal goal is to track the radius of the inner edge of the accretion disk in the hard state as a function of luminosity, as we have done before using RXTE data. Using NuSTAR advanced capabilities, we aim to resolve the gross discrepancies in published measurements of the inner disk radius.

PROPOSAL: 03127     PI: Sam Krucker

USING DISK-OCCULATION TO STUDY NON-THERMAL PROCESSES IN SOLAR ACTIVE REGIONS

    The aim of this proposal is to search for X-ray bremsstrahlung signatures produced by non-thermal electrons in the solar corona within active regions. Such electrons are thought to be accelerated by the release of coronal magnetic energy. The X-ray signatures of these events are predicted to be one of the smoking guns to identify the heating mechanism of the solar corona. Here we propose to use the solar disk as a natural occulter to shield NuSTAR from the intense thermal soft X-rays and observe non-thermal signatures from the corona. With the requested 80 ks (25 orbits), we are confident that we will catch several occulted microflares/nanoflares with at least one event seen during the impulsive rise phase.

PROPOSAL: 03195     PI: Thomas Nelson

UNDERSTANDING THE GAMMA-RAY PRODUCTION MECHANISM IN NOVA SHOCKS

    We request a 60 ks target-of-opportunity observation of the next gamma-ray-bright nova. The detection of gamma-rays from novae demonstrates that they are capable of accelerating particles to high energies. However, the site of particle acceleration, and the mechanism responsible for the gamma-ray emission, remains poorly understood. X-ray observations at E > 20 keV can probe the low-energy tail of the non-thermal emission that is detected by theFermi-LAT and provide important constraints on the physical conditions in the particle-accelerating shock. NuSTAR, with its unprecedented sensitivity above 10 keV, can therefore provide a unique probe of the shocks taking place in nova outbursts at early times.

PROPOSAL: 03200     PI: Giovanni Miniutti

EXPLORING THE JET-WIND CONNECTION IN MRK 231

    We propose for two 80 ks TOO observations of Mrk 231 with NuSTAR. We aim at obtaining two high quality NuSTAR X-ray spectra, one during a quiescent radio state, the other during a bright radio flare. Results from our previous radio and X-ray campaign show that low radio flux states are associated with the appearance of an X-ray ultra fast outflow (UFO) that is not detected during radio flares. This suggests that winds and jets could be mutually exclusive, as is the case in black hole X-ray binaries. We aim at confirming this behaviour with better statistics and at a different epoch to exclude chance coincidence. The resulting intermittent nature of UFOs in AGN, and the associated jet-wind anti-correlation will have an important impact on wind-driven AGN feedback models.

PROPOSAL: 03240     PI: Paul Scholz

SEARCHING FOR MAGNETAR-LIKE X-RAY BURSTS FROM THE REPEATING FAST RADIO BURST

    Fast Radio Bursts (FRBs) are a new class of astrophysical transients whose origins are currently a mystery. Early this year, we reported on the subarcsecond localization and identification of the host galaxy of the first, and to date only, repeating FRB, FRB 121102 (Chatterjee et. al 2017). Some models, notably those involving magnetars, predict hard X-ray bursts along with FRBs which may be detectable with NuSTAR. The emission of radio bursts from FRB 121102 seem to be highly clustered in time. We therefore propose a 100 ks NuSTAR Target of Opportunity observation with coordinated radio telescope observations to be performed in the event that FRB 121102 enters a period of radio burst activity.

PROPOSAL: 03296     PI: Joseph Neilsen

NUSTAR-NICER OBSERVATIONS OF BLACK HOLE TRANSIENTS

    With its excellent hard X-ray sensitivity, spectral resolution, and timing, NuSTAR has revolutionized our view of black hole accretion, particularly through studies of Compton scattering and relativistic reflection. But we still have yet to attain a fully self-consistent model of the accretion flow, in part because of the difficulty of constraining seed photons: thermal emission from cool, faint disks. With the launch of NICER in April 2017, we take a great step forward. NICER will be to soft X-rays what NuSTAR is to hard X-rays, and together they will produce incredibly tight constraints on the geometry of the inner disk, corona, and their interactions. We propose 5x20 ks coordinated observations of a black hole transient with NuSTAR and NICER to exploit these groundbreaking capabilities.

PROPOSAL: 04060     PI: Adam Ingram

TESTING RELATIVISTIC PRECESSION AROUND THE ACCRETING BLACK HOLE IN GRS 1915+105 WITH NUSTAR AND NICER

    We propose a 30 ks ToO, contemporaneous with a pre-approved 30 ks NICER ToO, to observe a low frequency quasi-periodic oscillation (QPO) from the accreting black hole (BH) GRS 1915+105. We will use QPO phase-resolved spectroscopy to test for QPO phase dependent changes in the shape of the reflection spectrum and disc temperature indicative of relativistic precession. The unique combination of NICER and NuSTAR will allow the first joint analysis of disc, iron line and reflection hump modulations. We will trigger from MAXI, plus Swift/XRT pointings, and aim to supplement our core science goals with multi-wavelength observations.

PROPOSAL: 04085     PI: Raffaella Margutti

DETAILED MAPPING OF EXTREME MASS LOSS WITH COORDINATED NUSTAR-XMM OBSERVATIONS

    Contrary to expectations from current stellar evolutionary models, recent observations uncovered the ejection of shells of material by massive stars in the years before the supernova (SN) explosion. The physical mechanism behind the impulsive mass ejection synchronized with the stellar core-collapse is unclear. Here we propose a coordinated NuSTAR-XMM effort to map the evolution of the broad-band X-ray spectrum of 1 nearby (d<50 Mpc) strongly interacting SN and enable progress. Our program has the immediate goal to characterize the medium around strongly interacting SN, which originate from stellar progenitors with the most extreme mass loss before explosion.

PROPOSAL: 04099     PI: Yanjun Xu

THE BRIGHT HARD STATE OF A TRANSIENT BLACK HOLE BINARY IS THE ACCRETION DISK TRUNCATED?

    We propose to undertake a 60 ks NuSTAR target-of-opportunity observation of a new or known black hole transient during a bright, hard outburst phase. The goals of this program are to 1) determine whether the accretion disk is truncated in the bright hard state via modeling the disk reflection spectrum 2) test an apparent anti-correlation between winds and jets in soft and hard states by making a very sensitive search for winds in a hard state 3) searching for quasi-periodic oscillations (QPOs) in the power spectrum and compare with theoretical models. This observation will be supported by a network of ground-based observatories, and X-ray monitoring.

PROPOSAL: 04101     PI: John Tomsick

THE SPIN RATES OF BLACK HOLES IN X-RAY BINARIES

    With its measurements of the reflection components from accretion disks in black hole (BH) binaries, NuSTAR has made significant progress in constraining BH spins. Determining BH spins is important for probing regions of strong gravity, investigating connections between spin and relativistic jets, and for comparing the X-ray binary spin distributions to those inferred for the BH-BH inspiral events. To continue this work, we propose a 50 ks TOO observation of any BH transient that has not been previously observed by NuSTAR. We will use Swift/XRT monitoring to trigger the NuSTAR observation if a BH transient shows evidence that it is making a transition from the hard state to the intermediate state.

PROPOSAL: 04147     PI: Teruaki Enoto

MAGNETIC ENERGY DISSIPATION IN MAGNETAR OUTBURSTS OBSERVED BY NUSTAR AND NICER

    We propose NuSTAR ToO observations (total 80 ks) of magnetar outbursts coordinated with the new soft X-ray timing instrument NICER. The evolution of the pulse profile and spectrum over the decay phase of a magnetar outburst provide crucial insight into the magnetic energy dissipation process in magnetars. Simultaneous coverage of the NICER (0.2-12 keV) and NuSTAR (3-79 keV) bands is essential for understanding the soft-thermal surface radiation and non-thermal magnetospheric emission. With the unprecedented sensitivity in the soft X-ray band of NICER, a pre-approved ToO can constrain the thermal evolution in the soft X-ray band. The addition of NuSTAR will allow simultaneous tracking of the hard X-ray flux -- a crucial element to understand the physics of magnetars.

PROPOSAL: 04161     PI: Nathalie Degenaar

TESTING DISK TRUNCATION FOR ACCRETING NEUTRON STARS

    It is commonly assumed that the accretion disk in low-mass X-ray binaries (LMXBs) moves significantly away from the black hole or neutron star when the X-ray luminosity decays below ~E-2 of the Eddington limit (Ledd). However, there is virtually no observational data to test this key aspect of accretion disk physics. Using a single 60-ks NuSTAR ToO observation and 11 ks of Swift/XRT monitoring, we propose to exploit the proximity and strong reflection features of the neutron star LMXB 4U 1608-52 to measure the location of its inner disk at a luminosity of ~E-3 Ledd. Comparing this with our previous NuSTAR observation obtained at E-2 Ledd, where we found the disk very close to the neutron star, we can directly test the disk truncation paradigm.

PROPOSAL: 04210     PI: Eric Bellm

NUSTAR CAN DISCOVER SPECTRAL COMPONENTS IN THE AFTERGLOWS OF ULTRA-LONG GRBS

    Ultra-long Gamma-Ray Bursts (ULGRBs) produce a gamma-ray transient more than several thousand seconds long followed by a decaying X-ray afterglow. Much remains uncertain about these recently-discovered events, most notably whether they are an extreme tail of normal long GRBs or represent a new class entirely. Our NuSTAR TOO observations of the afterglow of ULGRB 130925A unveiled a rich and unexpected phenomenology that may point towards the origin of these mysterious events. NuSTARs sensitivity and wide bandpass makes it the best instrument available to constrain multiple spectral components in GRB afterglows. We request one NuSTAR TOO for an ULGRB discovered in Cycle 4 to characterize its afterglow spectrum.

PROPOSAL: 04211     PI: Grzegorz Madejski

NUSTAR TOO OBSERVATIONS OF LUMINOUS BLAZARS

    We propose ToO observations of up to three flares of (a) 3 well-studied luminous blazars not previously studied with NuSTAR: 3C454.3, PKS1510-089, and PKS1222+216, and (b)one FSRQ blazar undergoing an exceptionally high gamma-ray flare in order to precisely measure their hard X-ray spectra, when the sources reach very high gamma-ray states as measured by Fermi-LAT. Once triggered, each source would be observed for 40 ks in a single time-unconstrained pointing, with the total request amounting to at most 120 ks. The NuSTAR observations will be supplemented with simultaneous data from Fermi and Swift (which routinely observes NuSTAR} targets to provide essential multiwavelength context and constrain the broad-band spectra.

PROPOSAL: 04285     PI: Lindsay Glesener

INVESTIGATION OF IMPULSIVE HEATING OF ACTIVE REGIONS IN THE SOLAR CORONA

    Our goal is to investigate the solar coronal heating problem, specifically the role of small solar flares in explaining how the Sun s corona is consistently heated to millions of degrees. To this aim, we will observe solar active regions with long enough integrations to detect faint signatures of small flares and will quantify their contributions to the heating of these particular regions. We will also combine NuSTAR measurements with those from novel solar-observing telescopes operating the extreme ultraviolet X-ray range to obtain the most sensitive measurements to date of flare heating of the corona. With these coordinated observations we will search for the weak high-temperature (>5MK) and non- thermal emission expected from impulsive energy release models.

PROPOSAL: 05066     PI: Sam Krucker

NUSTAR OBSERVATIONS OF QUIETEST X-RAY SUN: CORONAL HEATING AND AXIONS

    We propose to take the ultimate NuSTAR observations of the quiet Sun during the minimum of the solar cycle. These observations, collected at close to 100% livetime, will be used for the deepest-ever search for X-ray bremsstrahlung signatures to investigate the key question of coronal heating, and they will additionally allow us to search for an axion conversion signal. With solar activity being currently at a minimum and activity expected to pick up again later in 2020 or latest in 2021, the proposed observations are time-critical.

PROPOSAL: 05110     PI: Deanne Coppejans

NUSTAR OBSERVATIONS OF FAST AND BLUE STELLAR EXPLOSIONS

    We propose an in-depth X-ray study of fast blue optical transients (FBOTs), a new class of transients with luminosities and time scales that challenge traditional SN models. Alternative scenarios include a failed SN of a stripped star, He-shell detonation on a white dwarf, and a SN shock breaking through a dense medium. FBOTs have mostly been studied in the optical/UV regime, which is of thermal origin and it is not sensitive to the nature of the underlying energy source. Here we propose to capitalize on our recent NuSTAR discovery of hard X-rays from the FBOT AT2018cow with unprecedented properties, and start the first hard X-ray monitoring campaign of FBOTs. The primary goal is to test for the presence of engines driving the explosions and constrain their nature.

PROPOSAL: 05143     PI: Dominic Walton

THE HUNT FOR NEW PULSAR ULTRALUMINOUS X-RAY SOURCES: NGC7090 ULX

    Following a series of remarkable recent discoveries, we now know that some of the most luminous members of the ultraluminous X-ray source (ULX) population are actually powered by highly super-Eddington neutron stars. Five such systems are now known, but based on the long-term variability characteristics exhibited by these sources, we have compiled a sample of 19 additional ULX pulsar candidates from the broader ULX population. Here we propose a co-ordinated target of opportunity observation with XMM (100ks) and NuSTAR (100ks) of one of the most promising of these candidates, NGC7090 ULX, when the source is bright. Our main aims are to search for pulsations in order to test our prediction that this is another ULX pulsar, and constrain the broadband spectrum.

PROPOSAL: 05213     PI: John Tomsick

MONITORING A BRIGHT OUTBURST FROM A BLACK HOLE TRANSIENT

    Much of our understanding of the properties of accretion disks, relativistic jets, and the regions of strong gravity near black holes (BHs) come from observations of BH X-ray binaries. NuSTAR has made significant contributions to these studies especially through improved measurements of reflection components that have allowed us to access the inner disk and constrain the spins of BHs. Recently, comparisons between BH spins in X-ray binaries and in binary BH mergers have further increased the level of interest in BH spins, and a main focus of this proposal is to improve assessments of the uncertainties in BH spin measurements. We propose to do this by monitoring a bright BH transient with NuSTAR to systematically quantify the effect of the changing continuum on the BH spin measurement.

PROPOSAL: 05238     PI: Javier Garcia

THE PROPERTIES AND EVOLUTION OF ACCRETION DISKS IN BLACK HOLE BINARIES

    We propose a series of 4 Target of Opportunity (ToO) observations with NuSTAR and NICER for any of the five following recurrent black hole binaries: GX 339-4, H1743-322, 4U 1630-47, XTE J1550-564, or XTE 1752-223. We aim to obtain high signal-to-noise spectra of these sources at different accretion states as they go into outburst. Our principal goal is to use X-ray reflection spectroscopy to track the evolution of the inner edge of the accretion disk as a function of luminosity; estimate the spin of the black hole; and measure the accretion disk inclination, density, ionization and iron abundance. These observations will provide crucial constraints to support a large-scale data analysis program for these sources.

PROPOSAL: 05252     PI: Marianne Vestergaard

MONITORING AND TOO OF A RE-AWAKENING AGN: CATCHING THE MONSTER IN THE ACT!

    After a 10-year hiatus Mrk590 is rekindling its nuclear activity! Our Swift monitoring strongly suggests that the AGN is likely to turn on again soon. We wish to seize this rare opportunity to document, for the first time ever, the onset of AGN activity when it occurs since this can lead to significant insight on the long-standing issue of how AGNs are fueled. We ask to monitor Mrk590 bi-weekly with Swift and based thereon trigger a joint 3-visit program with NuSTAR and Swift. We aim to establish at which accretion luminosity the expected emission features of a standard accretion disk appear as the AGN turns on. This program is an important part of a broad-scoped, multi-wavelength study of this remarkable AGN.

PROPOSAL: 05284     PI: James Steiner

A CONTINUED NUSTAR & NICER LOOK AT REFLECTION AND THERMAL EMISSION IN CYGNUS X-1

    Understanding the corona and disk-coronal interaction is a forefront challenge in black hole systems. While NuSTAR is the leading instrument at probing the high-energy spectral reflection component, NICER provides a complementary view of the thermal accretion disk. Both components are very strong for Cyg X-1, but the disk is cool and out of reach for NuSTAR alone. This proposal is a continuation of a previously approved NuSTAR Cycle 3 program on Cyg X-1 using the uniquely powerful capability of combined continuum and reflection modeling provided with both instruments together. By chance, our first program covered the soft state only. We propose to expand and target hard states and request 3x20 ks NuSTAR (3x4 ks NICER) observations of Cyg X-1 for triggered monitoring of hard states.

PROPOSAL: 06027     PI: Sam Krucker

NuSTAR observations of quietest x-ray sun: coronal heating and axions

    We propose to take the ultimate NuSTAR observations of the quiet Sun during the minimum of the solar cycle. These observations, collected at close to 100% livetime, will be used for the deepest-ever search for X-ray bremsstrahlung signatures to investigate the key question of coronal heating, and they will additionally allow us to search for an axion conversion signal. With solar activity being currently at a minimum and activity expected to pick up again in 2021, the proposed observations are time-critical.

PROPOSAL: 06045     PI: Ralf Ballhausen

Searching for new cyclotron lines in transient pulsars

    Cyclotron resonant scattering features (CRSFs or cyclotron lines) are the only direct way to measure the B-field close to the surface of an accreting neutron star and probe the physics in the accretion column. Here we propose a 40ks ToO observation of a CRSF candidate source in outburst at a flux of 100mCrab or higher to discover new CRSF sources. The current sample of known CRSF sources underrepresents the huge parameter space of B-fields, luminosities and geometries and therefore any new discovery is valuable to provide insight in the physical conditions necessary to form an observable line. NuSTAR is the most sensitive instrument to date to discover new CRSFs and constrain their energy and profile.

PROPOSAL: 06074     PI: Jiachen Jiang

Studying the very high state of black hole transients using relativistic spectroscopy

    We propose a 30 ks NuSTAR ToO observation of a black hole X-ray binary in the very high state. With this observation, we will combine the two leading relativistic spectroscopy methods, relativistic reflection and continuum fitting, to make two simultaneous, independent measurements of the spin of the central BH. This combined approach is more powerful than either method applied individually, and has great potential for expanding our knowledge of BH formation and growth, accretion physics and strong gravity. In addition, an extended disk reflection model with a variable density parameter for the surface of the disk will be considered particularly.

PROPOSAL: 06086     PI: Enrico Bozzo

Unveiling the nature of newly discovered integral sources with NuSTAR and XMM

    We propose to carry out a 30 ks-long combined NuSTAR+XMM observation of any newly discovered INTEGRAL transient during the next observational cycle. As discussed in the proposal, these simultaneous high sensitivity broad-band data provide the most useful mean to unveil the nature of the transient and their timely scheduling is crucial to collect data from the source while it is still active in X-rays (typical outbursts of these unknown transients last from a week to about 10 days).

PROPOSAL: 06091     PI: Jon Miller

A look into the heart of a tidal disruption with NuSTAR

    X-rays probe fundamental aspects of tidal disruption events (TDE). We request a rapid 50 ks TOO observation of a TDE with a Seyfert-like flux in the NuSTAR band during Cycle 6. This flux and exposure will enable sensitive searches for the QPOs, disk reflection, winds, and reverberation that have been detected in prior TDEs. These tools can reveal the nature of nascent disks, super-Eddington disks, and inflow--outflow coupling. Studies of TDEs benefit from leveraging the best abilities of different observatories; the team is well positioned to support this program using Chandra, XMM-Newton, Swift, and HST.

PROPOSAL: 06121     PI: Victor Doroshenko

Follow-up of X-ray binaries discovered by eROSITA

    The eRosita X-ray survey started last year is expected to discover several millions of new sources and double the number of known Galactic X-ray binaries (XRBs). This will help to clarify several long-standing questions related to numbers, origin, and evolution of XRBs and massive stars in general, and accretion physics in low-luminosity XRBs. To fully realise this potential, follow-up observations in X-ray and other bands are, however, essential. Here we propose to start the exploration of this new dataset with NuSTAR follow-up of up to four select High mass X-ray binary (HMXB) candidates discovered by eRosita within current Cycle 6 period with the aim to confirm identifications, and study their broadband X-ray properties in detail.

PROPOSAL: 06133     PI: Ralf Ballhausen

Tracing spectral transitions toward low luminosities in neutron star high mass X-ray binaries

    High-energy radiation from highly-magnetized neutron stars at very low luminosities (<1e35 erg/s) has so far only been observed with NuSTAR in two sources, A 0353+26 and GX 304-1, which both have CRSFs above 45keV. The spectral shape at these low mass accretion rates is fundamentally different compared to higher luminosities and in particular, the formation of high-energy radiation is still an open question. The available data are, however, very incomplete, because such observations can only be performed for nearby sources. We therefore propose a 100ks ToO of a low luminosity HMXB but with intermediate CRSF energy to study the dependence of the high-energy radiation with magnetic field and improve our understanding of radiative transfer in magnetized plasmas.

PROPOSAL: 06156     PI: Felicia Krauss

Blazar X-rays and neutrinos: Investigating the most promising ICECUBE neutrino alerts

    We propose to perform up to two 40 ksec ToO follow-up observations of X-ray luminous blazars that we identify in association with two separate "gold" quality IceCube high-energy neutrino alerts. These alerts are likely to be astrophysical neutrinos. The first identification of an astrophysical source of high-energy neutrinos occurred in 2018, following IceCube-170922A and the BL Lac-type blazar TXS 0506+056. X-ray follow-up of likely-cosmic neutrino alerts has thus proven its utility for identifying possible neutrino counterparts, and most importantly to measure the hadronic contribution to the high-energy emission. We aim to use this approach to identify more such sources.

PROPOSAL: 06164     PI: Kirill Sokolovsky

Understanding the gamma-ray production mechanism in nova shocks

    We propose a 60ks observation for one new GeV bright nova to explore the particle acceleration and gamma-ray production mechanisms. We will constrain the particle acceleration efficiency by comparing thermal X-ray to GeV luminosity and search for predicted non-thermal X-rays. Understanding shocks in novae is relevant for other shock-powered transients including Type IIn supernovae, tidal disruption events and stellar mergers. NuSTAR is the only instrument capable of detecting X-rays from novae simultaneously with the GeV emission (the lower energy X-rays accessible to other focusing instruments are hidden by the nova shell at these times). The observations should be conducted now to take advantage of the simultaneous operations with Fermi/LAT.

PROPOSAL: 06165     PI: Mirko Krumpe

Rare and dramatic flux drops in eROSITA AGN: Accretion or line-of-sight obscuration changes?

    eROSITA, successfully launched in mid-2019, will perform multiple all-sky X-ray surveys. Monitoring roughly half a million AGN/quasars, eROSITA will identify rare, accretion depletion events as they occur. NuSTAR observations are needed to distinguish depletion from heavy obscuration events. In case of a depletion event we will explore how the X-ray corona respond to a sudden, major drop in accretion rate. In the case of the line of sight obscuration event, we will determine the amount of absorption. Thus, we propose four ToOs of 70 ks each.

PROPOSAL: 06166     PI: Andrea Gokus

Gamma-ray flares from supermassive black holes at high redshift

    We propose a 100 ks ToO observation of a high-redshift (z>=3) blazar during a gamma-ray flare detected by Fermi/LAT. Only 12 high-redshift blazars have been detected in gamma-rays. These objects are the most luminous and massive objects in the Universe, which allow to study the formation of active galactic nuclei in the early Universe. Because the high-energy hump of their SED peaks in the MeV range, X-ray and gamma-ray data is needed to constrain the hump's shape. We propose to get high-sensitivity X-ray data during enhanced gamma-ray activity of a blazar at high redshift. The data will be complemented by triggered radio observations. The resulting multiwavelength data set will yield important knowledge on the behaviour of flaring MeV blazars for future MeV telescopes.

PROPOSAL: 06197     PI: Dominic Walton

Testing the connection between the ultraluminous X-ray sources Holmberg IX X-1 and NGC1313 X-1

    Multi-epoch broadband spectroscopy of the ultraluminous X-ray source (ULX) NGC1313 X-1 has revealed unusual spectral variability. When fit with accretion disk models, the data show two distinct groups in the L-T plane, each of which exhibits its own positive L-T correlation. Understanding this behaviour is likely key to understanding the structure of super-Eddington accretion flows. Holmberg IX X-1 also shows some evidence for this behaviour, but there is currently only one broadband observation in the high-flux regime, meaning this cannot yet be firmly established. This proposal aims to take a series of 4 XMM+NuSTAR observations (each 25+50ks) of Holmberg IX X-1 in its high-flux state in order to determine whether it does show the same unusual behaviour as NGC1313 X-1.

PROPOSAL: 06209     PI: Georgios Vasilopoulos

Follow-up major outbursts from hard X-ray transients in the Magellanic Clouds

    The Magellanic Clouds (MCs) harbor a large sample of Be/X-ray binaries (BeXRBs) at a moderate and well known distance with low Galactic foreground absorption. We propose 3 ToO observations (100 ks total time) of new or unexplored BeXRBs in the MCs during super-Eddington outbursts. Our goal is to study their spectral and temporal properties to gain insight onto super-Eddington accretion.

PROPOSAL: 06214     PI: Sean Pike

A deeper look for pulsations from Circinus ULX5

    We propose a 100 ks NuSTAR observation of Circinus ULX5 in the high-flux state, triggered by a Swift XRT monitoring campaign in order to search for transient pulsations. The source is a bright, variable ULX in the Circinus galaxy, which is well-isolated from the nucleus and other X-ray sources, making it a prime ULX for further study. Although short observations of the source have been previously carried out, this would be the first deep exposure by NuSTAR which would allow for detection of pulsations at pulse fractions significantly lower than were previously possible. Additionally, it will yield superior constraints on spectral models, particularly at high energies. This observation will build upon the field's current understanding of ULX demographics, variability, and spectral modeling.

PROPOSAL: 06217     PI: Matteo Cerruti

NuSTAR observations of a new very-high-energy quasar

    We propose to perform NuSTAR observations of a new very-high-energy (VHE; E > 100 GeV) quasar, detected in a gamma-ray flaring state with the MAGIC array of Cherenkov telescopes. NuSTAR observations will be coordinated to happen simultaneously with MAGIC and will be part of a multi-wavelength campaign from optical to VHE gamma-rays. The goal of the project is to obtain a simultaneous multi-wavelength coverage of a bright flare from a VHE quasar, putting constraints on the acceleration and radiation mechanisms in relativistic jets from super-massive black holes.

PROPOSAL: 06222     PI: Andrea Sanna

Revealing accreting millisecond X-ray pulsars with simultaneous NuSTAR and NICER observations.

    We propose a ToO observation of Galactic X-ray transients aimed at discovering accreting millisecond X-ray pulsars. NuSTAR and NICER represent ideal observatories capable of fulfilling such goal, which will help increasing the knowledge of the evolutionary paths followed by NS, and possibly lead to breakthrough discoveries of extremely quickly rotating objects. We propose a 40 ks observations of newly discovered X-ray transients. Criteria set to maximise the probability of discovering new sources of this class, based on the X-ray flux (> 7 and 10 mCrab, in 2-10 and 20-40 keV energy bands, respectively), the spectral shape (a power law with index 1.5-2.5), and the absence of an alternative identification, will be used to trigger the proposed observations among newly discovered transients.

PROPOSAL: 06238     PI: Marianne Vestergaard

ToO observations of a re-awakening AGN: Exploring the physics of changing-look AGN

    After a 10-year break, Mrk 590 is rekindling its nuclear activity with a rise in X-ray flux! We wish to seize this rare opportunity to record, for the first time, the onset of AGN activity as it occurs since this can lead to significant insight on the long-standing issue of how AGNs are triggered and fueled. We ask to trigger up to 3 single-visit joint NuSTAR/Swift/XMM observations to determine if a standard thin accretion disk is being built up as the AGN turns on or if alternate physics are at play. Relatively inexpensive XMM exposures are needed to constrain any intrinsic absorption. The data are also ideal for testing the recent prediction that quasi-periodic eruptions can explain the flux changes seen for Mrk 590.

PROPOSAL: 06244     PI: Mallory Roberts

Catching the transition of the high B field redback PSR J2129-0429

    Redback millisecond pulsars serve as the link between radio millisecond pulsars and low-mass X-ray binaries, with several redbacks having transitioned from a radio pulsar state to an accretion state on timescales of months to years. PSR J2129-0429 is a gamma-ray and radio redback with an unusually high magnetic field (B ~ 1.8e9 G) and long pulse period (7.6 ms), suggesting it is relatively early in the recycling process. Observed changes in the color of the companion suggest it may transition to an LMXB state at any time, which would make it only the 4th redback to be observed transitioning. We propose to monitor the system with monthly Swift observations of ~ 3 ks in anticipation of a state change, at which point we would trigger an 100 ks NuStar TOO observation.

PROPOSAL: 07027     PI: Sam Krucker

USING DISK-OCCULTATION TO STUDY NON-THERMAL PROCESSES IN SOLAR FLARES WITH NUSTAR AND SOLAR ORBITER/STIX

    The aim of this proposal is to search for X-ray bremsstrahlung signatures, produced by non-thermal electrons within solar flares, in the solar corona. We propose to use the solar disk as a natural occulter ( coronagraph ) to shield NuSTAR from the intense flare emission lower down and thus reveal the much fainter non-thermal signatures from the corona. To also capture the main flare emissions that are not visible to NuSTAR, we will use Solar Orbiter STIX hard X-ray observations taken from a different vantage point than Earth. The time frame for observing occulted flares is ideal in Cycle 7, as solar activity is rising with moderately bright flares that are most suitable for NuSTAR solar observations.

PROPOSAL: 07037     PI: Murray Brightman

NUSTAR AND XMM-NEWTON OBSERVATIONS OF A NEWLY DISCOVERED ULX IN NGC 4045

    A new ULX in the galaxy NGC 4045 that reaches X-ray luminosities greater than 10^41 erg s^-1 was recently found, making it one of the most powerful ULXs discovered to date. DDT observations with NuSTAR revealed enticing candidate signals of an absorption line at 8.7 keV which could be the signature of an ultrafast outflow, the hallmark of super-Eddington accretion, and a potential pulsation signal, that could confirm the source as an neutron star accretor. However, neither of these were found to be significant in the 50-ks observation. Here we propose follow-up target of opportunity observations with NuSTAR and XMM-Newton in order to confirm these signals.

PROPOSAL: 07052     PI: Ralf Ballhausen

SEARCHING FOR NEW CYCLOTRON LINES IN TRANSIENT PULSARS

    Cyclotron resonant scattering features (CRSFs or cyclotron lines) are the only direct way to measure the B-field close to the surface of an accreting neutron star and probe the physics in the accretion column. Here we propose a 40ks ToO observation of a CRSF candidate source in outburst at a flux of 100mCrab or higher to discover new CRSF sources. The current sample of known CRSF sources underrepresents the huge parameter space of B-fields, luminosities and geometries and therefore any new discovery is valuable to provide insight in the physical conditions necessary to form an observable line. NuSTAR is the most sensitive instrument to date to discover new CRSFs and constrain their energy and profile.

PROPOSAL: 07106     PI: Raffaella Margutti

A BROAD-BAND X-RAY VIEW OF THE MOST EXTREME MASS-LOSS WITH NUSTAR+XMM

    Contrary to expectations from current stellar evolutionary models, recent observations uncovered the ejection of shells of material by massive stars in the years before the supernova (SN) explosion. The physical mechanism behind the impulsive mass ejection synchronized with the stellar core-collapse is unclear. Here we propose a coordinated XMM-NuSTAR effort to map the evolution of the broad-band X-ray spectrum of 1 nearby (d<50 Mpc) strongly interacting SN and enable progress. Our program has the immediate goal to characterize the medium around strongly interacting SN, which originate from stellar progenitors with the most extreme mass loss before explosion. This programs has the potential enlarge the very small sample of two strongly interacting SNe with broad-band X-ray detections.

PROPOSAL: 07112     PI: Yuhan Yao

NUSTAR OBSERVATIONS OF TIDAL DISRUPTION EVENTS

    A growing number of tidal disruption events (TDEs) are expected to be discovered by the ongoing Spektrum-Roentgen-Gamma (SRG) all-sky X-ray survey. We propose 2x40ks NuSTAR observations on two subgroups of SRG-discovered TDEs with hard X-ray emission. For non-relativistic TDEs with non-thermal X-ray emission, the observation will allow us to search for spectral features characteristic of relativistic reflection. For relativistic TDEs, this will provide the highest resolution hard X-ray spectrum ever obtained, shedding light on the power mechanism of TDE jets.

PROPOSAL: 07113     PI: Joseph Neilsen

NUSTAR-NICER OBSERVATIONS OF BLACK HOLE TRANSIENTS

    With its excellent hard X-ray sensitivity, energy resolution, and timing, NuSTAR has revolutionized our view of BH accretion, particularly through studies of Compton scattering and relativistic reflection. But we still have yet to attain a fully self-consistent model of the accretion flow, in part because of the difficulty of constraining seed photons: thermal emission from cool, faint disks. With the launch of NICER in June 2017, we took a great step forward. NICER is to soft X-rays what NuSTAR is to hard X-rays, and together they can produce incredibly tight constraints on the geometry of the inner disk, corona, and their interactions. We propose 5x20 ks coordinated observations of a black hole transient with NuSTAR and NICER (5x10 ks) to exploit their groundbreaking capabilities.

PROPOSAL: 07164     PI: Felicia McBride

BLAZAR X-RAYS AND NEUTRINOS: INVESTIGATING THE MOST PROMISING ICECUBE NEUTRINO ALERTS

    We propose to perform two 40 ksec target of opportunity follow-up observations of X-ray luminous sources that we identify in association with two separate ``gold''-quality IceCube high-energy neutrino alerts, which have a likelihood of at least 70% to be astrophysical in origin. The first identification of an astrophysical source of high-energy neutrinos occurred in 2018, following IceCube-170922A and the BL~Lac-type blazar TXS 0506+056. X-ray follow-up of likely-cosmic neutrino alerts has thus proven its utility for identifying possible neutrino counterparts, and most importantly to measure the hadronic contribution to the high-energy emission. We aim to use this approach to identify more such sources and calculate their neutrino flux.

PROPOSAL: 07179     PI: Mirko Krumpe

RARE AND DRAMATIC FLUX DROPS IN EROSITA-DETECTED AGN: CHANGES IN ACCRETION FLOW OR LINE-OF-SIGHT OBSCURATION?

    eROSITA is currently performing multiple all-sky X-ray surveys. Monitoring roughly 500 very bright AGN/quasars, eROSITA can identify rare, accretion shut-down events in these objects as they occur. NuSTAR observations are needed to distinguish shut-down from heavy occultation events. In case of a shut-down event, we will explore how the X-ray corona responds to a sudden, major drop in accretion rate. In the case of the line of sight occultation event, we will constrain cloud properties. The NuSTAR data are critical for the subsequent multi-wavelength follow-up strategy of already approved programs. Thus, we propose two ToOs of 100 ks each.

PROPOSAL: 07221     PI: Jean Somalwar

TIDAL DISRUPTION EVENTS IN ACTIVE GALACTIC NUCLEI

    Tidal disruption events in AGN can drive bright, slowly evolving optical flares. The few known TDE candidates in AGN lack extensive multi-wavelength follow up. In particular, both hard and soft X-ray observations are key to understanding how a TDE impacts the AGN disk and its X-ray corona. For example, a TDE is predicted to drive super-Eddington inflows which can destroy the AGN corona and result in a dramatic reduction in soft emission. Also, the collision of diffuse stellar debris with the AGN disk can produce hard X-ray flares. We propose a pilot program of joint NuSTAR+Swift observations of the most promising optically selected TDE-AGN candidate found in our vigorous program to discover such events in optical time-domain surveys, to probe physical conditions in their innermost regions.

PROPOSAL: 07259     PI: Missagh Mehdipour

TRACKING TRANSIENT OBSCURING OUTFLOWS IN AGN

    Obscuring outflows are remarkably different from the common warm-absorber outflows in AGN. They exhibit large columns of high-velocity gas close to the black hole. They shield much of the X-ray radiation, which can have important implications for the surrounding gas and the launching of outflows. Joint ToO observations with NuSTAR and XMM, triggered using Swift monitoring, have been instrumental for the new results on obscuration events to come into light. We propose to investigate a new event to help us address current questions regarding the obscuring outflows and broaden our understanding of these transient events in AGN. We request a ToO NuSTAR observation (50 ks) to be taken jointly with a XMM-Newton observation (50 ks), using weekly Swift monitoring of an AGN sample (93 ks in total).

PROPOSAL: 07266     PI: Benjamin Coughenour

SPIN AND REFLECTION IN A BLACK HOLE TRANSIENT

    We propose a 50 ks NuSTAR TOO observation of a transient black hole (BH) source during outburst. Using Swift/XRT to determine the source state, we will trigger the NuSTAR observation when the source transitions from the hard state to its intermediate state. Modeling of reflection features in the source spectrum will be used to measure the BH spin, as well as constrain the environment of the innermost accretion disk. We will only target sources that have not yet been observed by NuSTAR during outburst, whether that means an entirely new BH candidate or a previously known BH source that has been in quiescence since the launch of NuSTAR. Examples of such sources include GRO J1655-40 and 4U 1543-475, allowing us to test different methods for measuring the black hole spin.

PROPOSAL: 07286     PI: Qi Feng

SEARCHING FOR NEUTRINO-EMITTING BLAZARS IN HARD X-RAY BAND

    We propose NuSTAR target-of-opportunity (ToO) observations of a candidate neutrino-emitting blazar, triggered by the combination of an IceCube gold alert and detections of a spatially coincident blazar by Fermi-LAT and Swift-XRT. We request 40 ks of NuSTAR observations, limited to one trigger, scheduled as soon as possible after the trigger.

PROPOSAL: 08033     PI: Ralf Ballhausen

SEARCHING FOR NEW CYCLOTRON LINES IN TRANSIENT PULSARS

    Cyclotron resonant scattering features (CRSFs or cyclotron lines) are the only direct way to measure the B-field close to the surface of an accreting neutron star and probe the physics in the accretion column. Here we propose a 50ks ToO observation of a CRSF candidate source in outburst at a flux of 100mCrab or higher to discover new CRSF sources. The current sample of known CRSF sources underrepresents the huge parameter space of B-fields, luminosities and geometries and therefore any new discovery is valuable to provide insight in the physical conditions necessary to form an observable line. NuSTAR is the most sensitive instrument to date to discover new CRSFs and constrain their energy and profile.

PROPOSAL: 08041     PI: Jiachen Jiang

STUDYING THE VERY HIGH STATE OF BLACK HOLE TRANSIENTS USING RELATIVISTIC SPECTROSCOPY

    We propose a 30 ks NuSTAR ToO observation of a black hole X-ray binary in the very high state. With this observation, we will combine the two leading relativistic spectroscopy methods, relativistic reflection and continuum fitting, to make two simultaneous, independent measurements of the spin of the central BH. This combined approach is more powerful than either method applied individually, and has great potential for expanding our knowledge of BH formation and growth, accretion physics and strong gravity. In addition, an extended disk reflection model with a variable density parameter for the surface of the disk will be considered particularly.

PROPOSAL: 08079     PI: Benjamin Coughenour

SPIN AND REFLECTION IN A BLACK HOLE TRANSIENT

    We propose a joint 50 ks NuSTAR and 5 ks NICER TOO observation of a transient black hole (BH) X-ray binary source during outburst. Using NICER monitoring to determine the source state, we will trigger the primary joint NuSTAR/NICER observation when the source transitions from the hard state to its intermediate state. Modeling reflection features in the source spectrum will be used to measure the BH spin, as well as constrain the innermost environment of the accretion disk. We will only target sources that have not yet been observed by NuSTAR during outburst, whether that means an entirely new transient BH candidate or a previously known BH X-ray binary that has been in quiescence since the launch of NuSTAR. Examples of well-known sources include GRO J1655-40 and XTE J1550-564.

PROPOSAL: 08110     PI: Vaidehi Paliya

UNRAVELING THE SUPERLUMINOUS BLAZAR FLARES WITH NUSTAR

    We propose to trigger a maximum of two, each 20 ksec, target of opportunity (ToO) observations of z>2 Swift-BAT detected blazars when they exhibit GeV flares (0.1-300 GeV flux > 1e-6 ph/cm2/s). The primary objective is to systematically characterize, for the very first time, the underlying radiating particle behavior responsible for the flare, determine the total jet power, and also constrain the Doppler boosting factor of high-redshift blazars during their extraordinary GeV flaring episodes by harnessing the unprecedented capabilities of NuSTAR. Since gamma-ray flares from high-z blazars typically last for a week, these ToO observations can easily be carried out within a week of the trigger, thus enabling us to extract a maximum science with a minimum impact on the satellite operation.

PROPOSAL: 08121     PI: Samaresh Mondal

CHARACTERIZING X-RAY TRANSIENTS IN THE GC USING NUSTAR

    We propose a 30 ks NuSTAR ToO observation to follow-up a new faint or very faint X-ray transient in the central Galactic disk region. Using Swift/XRT to determine the source state, we will trigger the NuSTAR observation when the source is in the bright outburst phase. Our goal is to test various spectral models for the continuum and constrain the high energy spectral cut-off, which will help us understand the nature of the source and, for accreting compact objects, the properties of the compact inner region (electron temperature). Furthermore, we would search for the reflection component in the spectra which can be used to probe the environment of the innermost accretion disk.

PROPOSAL: 08136     PI: Kirill Sokolovsky

UNDERSTANDING THE GAMMA-RAY PRODUCTION MECHANISM IN NOVA SHOCKS

    We propose a 60ks observation of a new bright (V < 7.5) nova likely to be detected in gamma-rays. The observation will probe shocks within the nova ejecta, constrain the non-thermal particle acceleration and gamma-ray production mechanisms. We will put an upper limit on the particle acceleration efficiency by comparing thermal X-ray and optical to GeV luminosity and search for predicted non-thermal X-rays. Understanding shocks in novae is relevant for other shock-powered transients including Type IIn supernovae, tidal disruption events and stellar mergers. NuSTAR is the only instrument capable of detecting hard X-rays from novae simultaneously with the GeV emission. The observations should be conducted now to take advantage of the simultaneous operations with Fermi and XMM.

PROPOSAL: 08166     PI: Qi Feng

SEARCHING FOR NEUTRINO-EMITTING BLAZARS IN HARD X-RAY BAND

    We propose NuSTAR target-of-opportunity (ToO) observations of a candidate neutrino-emitting blazar, triggered by the combination of an IceCube neutrino alert and detections of a spatially coincident blazar by Fermi-LAT and Swift-XRT. We request 40 ks of initial NuSTAR observation within 24 hours, on a best-effort basis, after the trigger. If the hard-X-ray flux from the initial observation is > 1.5 x 10^-12 erg cm^-2 s^-1, we request another 40 ks of follow-up NuSTAR observations to characterize the low-flux state.

PROPOSAL: 08171     PI: Felicia Mcbride

BLAZAR X-RAYS AND NEUTRINOS: INVESTIGATING THE MOST PROMISING ICECUBE NEUTRINO ALERTS

    We propose to perform two 40 ksec target of opportunity follow-up observations of X-ray luminous sources that we identify in association with two separate 'gold-quality' IceCube high-energy neutrino alerts. These alerts are likely to be from astrophysical neutrinos. The first identification of an astrophysical source of high-energy neutrinos occurred in 2018, following IceCube-170922A and the BL Lac-type blazar TXS 0506+056. X-ray follow-up of likely-cosmic neutrino alerts has thus proven its utility for identifying possible neutrino counterparts, and most importantly to measure the hadronic contribution to the high-energy emission. We aim to use this approach to identify more such sources and calculate their neutrino flux.

PROPOSAL: 08201     PI: Aarran Shaw

PROBING RAPID VARIABILITY IN BLACK HOLE X-RAY BINARY JETS

    Despite ~50 years of compact object studies in the Galaxy, the physics of relativistic jet launching remains an active and open field of research. Luckily, with the recent successful launch of JWST, a new parameter space is opening up in the field of jet-physics, allowing for the study of the spectral break above which the jet becomes optically thin on sub-second time scales, providing a crucial link between light and plasma properties. We are proposing to use the unique combination of high time resolution and hard X-ray coverage of NuSTAR to study the outburst of two BH-LMXBs in a jet-dominated hard state. We will probe evolving, rapid, sub-second variations, and, for the first time ever, we will search for correlated variability with mid-infrared spectral observations with JWST.

PROPOSAL: 08211     PI: Axel Arbet-Engels

NUSTAR FOLLOW-UP OBSERVATIONS OF EXCEPTIONAL EVENTS IN HARD-TEV BL LAC TYPE OBJECTS

    We propose a deep NuSTAR follow-up observation of 40ks on a bright VHE gamma-ray flare from a hard-TeV BL Lac object detected by the MAGIC Telescopes. Our goal is to build unprecedented broadband spectral energy distribution in order to unveil the underlying physical processes from these blazars, which hard-TeV spectrum represent evident challenges for standard emission models.

PROPOSAL: 08219     PI: Paul Draghis

MEASURING THE SPIN OF FUTURE BLACK HOLE TRANSIENTS

    The distribution of spins across stellar mass black holes (BH) is currently not well understood. NuSTAR is the instrument best suited for spin measurements of BH in X-ray binaries using relativistic reflection. We propose dedicating 60 ks spread in two 30 ks observations to four upcoming, previously unobserved by NuSTAR, BH transients with fluxes above 50 mCrab. For this flux threshold, a 30ks observation would suffice to measure the BH spin, while two observations taken ~15 days apart in different spectral states allow probing the evolution of the system. This treatment followed over the next observing cycles will maximize the number of new BHs observed and, together with gravitational wave observations, will allow developing a unifying view of the distribution of stellar mass BHs.

PROPOSAL: 08226     PI: Sean Pike

RAPID FOLLOW-UP OF UNKNOWN MAXI SOURCES NEAR THE SUN WITH NUSTAR

    We propose rapid follow-up observations of up to 3 unknown MAXI X-ray transients which lie between 10 and 47 degrees of the Sun using NuSTAR for in order to tile the corresponding MAXI error region, localize the source, and perform spectral analyses for the purpose of source classification. The purpose of these observations is to cover a part of the sky which is inaccessible to other X-ray observatories and therefore to serve the community by performing important groundwork to facilitate further study of X-ray sources such as low mass X-ray binaries.

PROPOSAL: 08227     PI: Axel Arbet-Engels

SIMULTANEOUS X-RAY AND VHE OBSERVATIONS TO UNVEIL THE INTERMITTENT EXTREME NATURE OF 1ES 2344+514

    We propose to study the blazar 1ES2344+514 with a total of 40ks of NuSTAR observations with simultaneous soft X-ray and VHE observations. 1ES 2344+514 is considered as an intermittent extreme BL Lac type object. Observations suggest that the synchrotron peak of the spectral energy distribution shows an impressive shift to higher energies by about two orders of magnitude during flares, reaching the hard X-ray band. So far, such an extreme behaviour was not found during its low activity, but existing observations do not allow for a coherent picture of the complex spectral behaviour. By allowing us to track the evolution of the electron distribution, NuSTAR will provide insights in the acceleration and cooling processes, likely to be at the origin of the intermittent extreme nature.

PROPOSAL: 08233     PI: Daniel Lawther

TOO OBSERVATIONS OF A RE-AWAKENING AGN: EXPLORING THE PHYSICS OF CHANGING-LOOK AGN

    After a 10-year hiatus, Mrk 590 has partially re-ignited, with repeated major X-ray and UV flare-ups captured by Swift monitoring. We wish to seize this rare opportunity to document the onset of AGN activity as it occurs, since this can lead to significant insight on the long-standing issue of how AGNs are triggered and fueled. We will use approved Swift XRT monitoring to trigger up to 4 single-visit NuSTAR observations, of which up to 3 are joint with XMM (or NICER). Together, these data are needed to determine if a standard thin accretion disk is being built up as the AGN turns on (and then characterize this event) or if alternate physics are at play.

PROPOSAL: 09029     PI: Kaya Mori

A HARD X-RAY SURVEY OF POLARS PROBING WHITE DWARF MAGNETIC FIELD FORMATION

    The proposed program will conduct the first extensive hard X-ray survey of polars by networking with optical transient monitors and AAVSO. Polars, a class of magnetic CVs with a wide range of B-fields (7-240 MG), offer a unique way to probe the origin of white dwarf (WD) B-fields. We aim to measure WD masses of 15 polars to <20% error using NuSTAR's broadband X-ray spectra and the most advanced X-ray spectral model available for polars. An extensive data set of well-measured WD masses and B-fields will test one of the fundamental predictions from the leading common envelope theory of CV formation. Furthermore, NuSTAR timing data will provide the most sensitive search for long-sought X-ray QPOs and elucidate an unresolved problem on the stability of a magnetic accretion shock.

PROPOSAL: 09047     PI: Mariko Kimura

MULTI-WAVELENGTH OBSERVATIONS DURING XRISM PV PHASE FOR THE DWARF NOVA SS CYGNI

    We propose NuSTAR observations of SS Cyg during the XRISM PV phase (100~ks during outburst and 30~ks during quiescence). SS Cyg is suitable for investigating X-ray emitting hot plasma in the vicinity of compact objects and accretion physics universal in many astronomical objects with accretion disks. Hard X-ray observations with NuSTAR are indispensable for accurately modeling complex emission lines from the plasma, which is the key to achieving the immediate objectives of the XRISM science team. We also perform coordinated radio, near-infrared, and optical observations during outbursts to detect evidence of jet ejections. The proposed observation will help to determine the plasma geometry and the WD mass and elucidate the radio emission's origin during an outburst.

PROPOSAL: 09050     PI: Sam Krucker

USING DISK-OCCULTATION TO STUDY NON-THERMAL PROCESSES IN SOLAR FLARES WITH NUSTAR AND SOLAR ORBITER/STIX

    The aim of this proposal is to search for X-ray bremsstrahlung signatures, produced by non-thermal electrons within solar flares, in the solar corona. The X-ray signatures of these events are the smoking guns necessary to understand magnetic energy release and particle acceleration in solar flares. These events can generally not be detected as they are overwhelmed by the much brighter X-ray emissions from the chromosphere. Here we propose to use the solar disk as a natural occulter ( coronagraph ) to shield NuSTAR from the intense flare emission lower down and thus reveal the much fainter non-thermal signatures from the corona. To also capture the main flare emissions, we will use Solar Orbiter STIX hard X-ray observations taken from a different vantage point than Earth.

PROPOSAL: 09085     PI: Sibasish Laha

TRACKING THE EVOLUTION OF CORONAL PROPERTIES IN A CHANGING LOOK AGN

    Large-scale time-domain surveys have lead to the identification of new types of extreme variability in active galaxies, called changing look active galactic nuclei (CL-AGNs). Drastic changes in coronal emission have been seen in CL-AGNs at different timescales (factor of 10-100 in flux and spectral slope ($\Gamma=1-5$) in a matter of months/years), thus providing an ideal laboratory to understand the fundamental physics behind coronal emission. Here we propose to observe a CL-AGN with Nustar for 40ks, once every month for 4 months, to measure the X-ray cut-off energy, which may be evolving during the highly variable state. We will therefore be able to constrain any changes in the thermal/non-thermal fraction in the coronal plasma in real time, for the first time.

PROPOSAL: 09094     PI: Joseph Neilsen

NUSTAR-NICER OBSERVATIONS OF BLACK HOLE TRANSIENTS

    With its excellent hard X-ray sensitivity, energy resolution, and timing, NuSTAR has revolutionized our view of BH accretion, particularly through studies of Compton scattering and relativistic reflection. But we still have yet to attain a fully self-consistent model of the accretion flow, in part because of the difficulty of constraining seed photons: thermal emission from cool, faint disks. With the launch of NICER in June 2017, we took a great step forward. NICER is to soft X-rays what NuSTAR is to hard X-rays, and together they can produce incredibly tight constraints on the geometry of the inner disk, corona, and their interactions. We propose 5x20 ks coordinated observations of a black hole transient with NuSTAR and NICER (5x10 ks) to exploit their groundbreaking capabilities.

PROPOSAL: 09108     PI: Reshmi Mukherjee

SEARCHING FOR NEUTRINO-EMITTING BLAZARS IN HARD X-RAY BAND

    We propose NuSTAR target-of-opportunity (ToO) observations of a candidate neutrino-emitting blazar, triggered by the combination of an IceCube neutrino alert and detections of a spatially coincident blazar by Fermi-LAT and Swift-XRT. We request 40 ks of initial NuSTAR observation within 24 hours, on a best-effort basis, after the trigger. If the hard-X-ray flux from the initial observation is > 1.5 x 10^-12 erg cm^-2 s^-1, we request another 40 ks of follow-up NuSTAR observations to characterize the low-flux state.

PROPOSAL: 09109     PI: Dheeraj Pasham

NUSTAR AND NICER OBSERVATIONS TO IDENTIFY AND STUDY COSMOLOGICAL BLACK HOLES AS THEY TURN ON A RELATIVISTIC JET

    Following NICER+NuSTAR's recent success in capturing the spectro-timing variability of the farthest stellar tidal disruption event (TDE) to-date we propose ToO monitoring observations of a future relativistic TDE, i.e., a system with a newborn relativistic jet pointed directly along our line of sight. Our main goals are 1) to establish the relativistic nature of the future transient by measuring its luminosity and variability, and 2) combine NICER+NuSTAR data with our approved multi-frequency radio data to perform multi-epoch spectral energy distribution modelings to shed light on the underlying jet physics. Our scientific goals require high-cadence monitoring for weeks and hard X-ray coverage making NICER + NuSTAR ideal facilities for the proposed study.

PROPOSAL: 09124     PI: Amruta Jaodand

CHASING THE X-RAY AFTERGLOWS OF GRAVITATIONAL-WAVE EVENTS

    Many astrophysical questions regarding NS mergers still remain open. What fraction of them have central engines and how long do they operate? What is the maximum mass for a stable NS remnant? How much energy do mergers release? Do all mergers produce successful relativistic jets and short GRBs? Broadband X-ray observations (Using soft and hard X-ray observations) of diverse NS merger events (especially in conjunction with radio observations) will be able to answer many of these questions. In LIGO O4 we expect about 2--7 GW events with electromagnetic counterparts, and therefore arc-second localizations. NuSTAR's broadband coverage and ability to look close-to/at the Sun will crucial for following-up these neutron star gravitational wave mergers.

PROPOSAL: 09138     PI: Marina Orio

A SYMBIOTIC NOVA IN OUTBURST

    Symbiotic novae are the best, and probably the only site to measure the prompt hard X-ray emission of novae and the evolution of the hottest component in their shocked ejecta. They are due to thermonuclear runaways on a white dwarf accreting from a red giant or AGB companion, and emit copious flux in all the electromagnetic spectrum, from gamma-rays to radio. Observing a symbiotic nova in outburst with NuSTAR will measure the primary shock that accelerates particles, map the spectral energy distribution after gamma-rays originate, then follow the trends of the maximum plasma temperature and cooling rate. Following the shock evolution will also significantly constrain the models and yield parameters to derive chemical yields.

PROPOSAL: 09180     PI: Megumi Shidatsu

UNDERSTANDING DISK WINDS IN TRANSIENT BLACK HOLE X-RAY BINARIES WITH A COORDINATED XRISM AND NUSTAR OBSERVATION

    We propose a joint XRISM and NuSTAR ToO observation of the disk wind in a Galactic transient black hole X-ray binary. The main science goal is to understand the launching mechanism of disk winds and feedback to the environment. XRISM high-resolution spectroscopy enables us to resolve absorption lines from winds and obtain key parameters of the conditions of the outflowing gas, such as the ionization level, the column density, and the velocity structure. Combining the NuSTAR broadband spectrum extending to the hard X-ray band, we can accurately estimate the launch site of the observed wind and that predicted for thermally driven winds. Using this information, we can also estimate the mass loss rate of the wind.

PROPOSAL: 09191     PI: Svetlana Jorstad

COMBINING X-RAY POLARIZATION AND CONTINUUM SPECTRA OF BLAZARS

    The investigators propose to observe two X-ray flaring blazars with NuSTAR and Swift during X-ray linear polarization measurements by the Imaging X-ray Polarimetry Explorer (IXPE). The combined simultaneous 0.3-70 keV continuum spectrum and polarization derived from synchronous NuSTAR, Swift, and IXPE observations, along with mm-wave and optical polarization measurements, will provide a definitive test of the synchrotron self-Compton and external Compton models for X-ray emission from low-synchrotron-peak blazars, and for the hard X-ray emission from intermediate-synchrotron-peak blazars.

PROPOSAL: 09198     PI: Kirill Sokolovsky

UNDERSTANDING THE GAMMA-RAY PRODUCTION IN NOVA SHOCKS

    We propose a 60ks observation of a new bright (V<7.5) nova likely to be detected in gamma-rays. The observation will probe shocks within the nova ejecta, constrain the non-thermal particle acceleration and gamma-ray production mechanisms. We will put an upper limit on the particle acceleration efficiency by comparing thermal X-ray and optical to GeV luminosity and search for predicted non-thermal X-rays. Understanding shocks in novae is relevant for other shock-powered transients including Type IIn supernovae, tidal disruption events and stellar mergers. NuSTAR is the only instrument capable of detecting hard X-rays from novae simultaneously with the GeV emission. The observations should be conducted now to take advantage of the simultaneous operations with Fermi and XMM.

PROPOSAL: 09235     PI: Aarran Shaw

PROBING RAPID VARIABILITY IN BLACK HOLE X-RAY BINARY JETS

    Despite ~50 years of compact object studies in the Galaxy, the physics of relativistic jet launching remains an active and open field of research. Luckily, with the successful launch of JWST, a new parameter space is opening up in the field of jet physics, allowing for the study of the spectral break - above which the jet becomes optically thin - on sub-second time scales, providing a crucial link between light and plasma properties. We are proposing to use the unique combination of high time resolution and hard X-ray coverage of NuSTAR to study the outburst of two BH-LMXBs in a jet-dominated hard state. We will probe evolving, rapid, sub-second variations, and, for the first time ever, we will search for correlated variability with mid-infrared spectral observations with JWST.

PROPOSAL: 09251     PI: Koji Mukai

THE MASSIVE WHITE DWARF IN THE SYMBIOTIC STAR T CRB: GRAVITATIONAL RESDSHIFT VS. CONTINUUM TEMPERATURE

    The symbiotic recurrent nova T Corona Borealis (T CrB) likely harbors a massive white dwarf near the Chandrasekhar limit. However, the methods that have been used to estimate the white dwarf mass are potentially subject to systematic errors. The forthcoming X-ray observatory, XRISM, with its microcalorimeter instrument, Resolve, has the potential to allow a direct measurement of the gravitational redshift on the white dwarf surface, and hence its mass. Here we propose a coordinated NuSTAR observation to support this by providing an accurate characterization of the broad-band X-ray spectrum of T CrB, and by providing a direct comparison with an X-ray continuum-based estimate of the white dwarf mass.

PROPOSAL: 09257     PI: Sean Pike

RAPID FOLLOW-UP OF UNKNOWN MAXI SOURCES NEAR THE SUN WITH NUSTAR

    We propose rapid follow-up observations of up to 3 unknown MAXI X-ray transients which lie between 10 and 47 degrees of the Sun using NuSTAR in order to tile the corresponding MAXI error region, localize the source, and perform spectral analyses for the purpose of source classification. Sources near the Sun often go unobserved due to the limitations of many X-ray observatories. The ToO program we propose will ensure that observations are carried out for sources that would be unobservable to observatories other than NuSTAR. As demonstrated by a number of recent NuSTAR observations at low Sun angle, observations like this have the potential to help answer important astrophysical questions regarding the evolution of accretion disks throughout outbursts, the onset of jet formation, and more.

PROPOSAL: 09271     PI: Jennifer Sokoloski

A HARD X-RAY VIEW OF THE BRIGHTEST NOVA THIS CENTURY

    The goal of this ToO proposal is to use high S/N hard X-ray spectra from NuSTAR and NICER during the upcoming eruption of the symbiotic recurrent nova T CrB to test two models for the GeV and TeV emission from such novae. We will determine whether novae in symbiotic binary stars lead to multiple gamma-ray-producing shocks that traverse and probe different portions of the ejecta and/or circumbinary medium. The strength of non-thermal X-ray emission will also constrain the origin of gamma-rays. T CrB, which may experience its third ever nova eruption during NuSTAR AO9, is the ideal source for this test, the results of which have important implications for the physics of particle acceleration in shocks, high-energy emission from eruptive transients, and mass transfer in wide binaries.

PROPOSAL: 09274     PI: Oluwashina Adegoke

UNDERSTANDING OBSCURATION IN DIPPING BLACKHOLE BINARIES

    A few blackhole X-ray binaries (BHXBs) have been known to show recurrent intensity dips in their lightcurves during outburst. These systems also show indication of outflowing winds. The dips are believed to be caused by clumps of cold obscuring material. Until now, our knowledge of the properties of such obscurer and its relation with disk winds as well as spectral state is very limited. Because soft X-ray photons are mostly absorbed by the obscuring material, NuSTAR's high energy capabilities make it the instrument of choice to study the nature of dips and obscuration in BHXBs. We therefore seek Target of Opportunity (ToO) observations with NuSTAR, jointly with NICER, for any one of a selected list of seven known dipping BHXBs in case they go into outburst during cycle 9.

PROPOSAL: 09276     PI: Javier Garcia

MEASURING BLACK HOLE SPIN AND MASS THROUGH X-RAY REFLECTION SPECTRA AND REVERBERATION LAGS

    X-ray reflection in accreting black holes occurs in the inner region of the accretion disk, and physical modeling of the spectral and timing properties of this emission enables measurement of the black hole mass and spin. The unique combination of NuSTAR s hard and NICER s soft coverage provides the broad bandpass, high count rates and energy and timing resolution required to constrain models of the time averaged energy spectrum and the reverberation lag energy spectrum on different timescales. We propose to observe any black hole transient, known or unknown, exceeding 30 mCrab during the bright hard state for 30 ks with a simultaneous NuSTAR and NICER observation in order to access unprecedented characterization of black hole spin and mass.

PROPOSAL: 09289     PI: James Steiner

CONTINUUM-FITTING SPIN MEASUREMENT OF A BLACK-HOLE TRANSIENT

    Stellar-mass black holes in X-ray transients undergo months-long outbursts during which they explore wide-ranging accretion rates and spectral-timing states. Following transitioning to the thermal/soft state, the black hole undergoes a protracted thermal decline. The thermal/soft state includes only modest contribution from nonthermal components; instead, most of the emission comes from the thermal accretion disk continuum. Accordingly, this state is the gold-standard for spin measurements via X-ray continuum fitting. We request 4x20 ks NuSTAR observations during the thermal phase of the outburst, to monitor a bright black-hole transient in decline, in order to determine its spin. We request a corresponding set of 4x5 ks NICER observations, which significantly improves the constraint.

PROPOSAL: 10066     PI: Jennifer Sokoloski

A HARD X-RAY VIEW OF THE BRIGHTEST NOVA THIS CENTURY

    The goal of this ToO proposal is to use high S/N hard X-ray spectra from NuSTAR and NICER during a nova eruption of the symbiotic recurrent nova T CrB to test two models for the GeV and TeV emission from such events. We will determine whether novae in symbiotic binaries lead to multiple gamma-ray shocks that traverse and probe different portions of the ejecta and circumbinary medium. The strength of non-thermal X-ray emission will also constrain the origin of gamma-rays. T CrB, which may experience a nova during Cycle 10, is ideal for this test, the results of which have implications for the physics of particle acceleration in shocks, high-energy emission from eruptive transients, and mass transfer in wide binaries. A project with this objective was approved but not triggered in Cycle 9.

PROPOSAL: 10087     PI: Gaurava Jaisawal

THERMONUCLEAR X-RAY BURSTS FROM 4U 1820-30

    We propose to observe thermonuclear X-ray bursts from 4U 1820-30 in its low-hard state using NuSTAR and NICER. The goal of this study is to understand the nature of the so-called persistent emission and how it responds to X-ray bursts. We aim to investigate the timing and spectral properties of the bursts, including their recurrence time. Using various spectral models, our study will be crucial in examining how photospheric radius expansion bursts affect the geometry of the accretion flow, such as the inner disk and corona.

PROPOSAL: 10090     PI: Kaya Mori

NUSTAR TOO OBSERVATIONS OF X-RAY TRANSIENTS IN THE GALACTIC CENTER

    We propose NuSTAR ToO observations of new X-ray transients and recurrent outbursts from the known very faint X-ray transients (VFXTs) through daily Swift-XRT monitoring of the Galactic Center. Broad-band X-ray spectral and timing data obtained by NuSTAR will provide useful diagnostic tools for identifying X-ray transients in the Galactic Center. NuSTAR will play a crucial role in investigating the nature of X-ray transients in the central parsec, as demonstrated by the discovery of a transient magnetar, the black hole spin measurements from two new transients, and characterizing VFXT's broadband X-ray properties. Identifying new X-ray transients, combined with investigating quiescent X-ray binaries, will explore the unique population and formation of X-ray binaries near Sgr A*.

PROPOSAL: 10097     PI: Raffaella Margutti

PROMPT BROAD-BAND FOLLOW-UP OF THE MOST NEARBY STRONGLY-INTERACTING SNE WITH NUSTAR+XRT

    Contrary to expectations from current stellar evolutionary models, recent observations uncovered the ejection of shells of material by massive stars in the years before the supernova (SN) explosion. The physical mechanism behind the impulsive mass ejection synchronized with the stellar core-collapse is unclear. Building on the recent success with SN2023ixf, here we propose a coordinated NuSTAR-XRT effort to map the evolution of the broad-band X-ray spectrum of 1 young nearby (d< 20 Mpc) or X-ray bright (Fx>5 x 10^-14 erg/s/cm2) strongly interacting SN and enable progress. Our program has the immediate goal to characterize the medium around strongly interacting SNe, which originate from stellar progenitors with the most extreme mass loss before explosion.

PROPOSAL: 10100     PI: Raffaella Margutti

A SYSTEMATIC SEARCH FOR HARD X-RAYS FROM COMPACT-OBJECT MERGERS

    We propose NuSTAR observations of the high-energy afterglows of GW sources discovered during Cycle 10 (which overlaps with the second half of LIGO/Virgo/KAGRA Observing Run 4) to constrain the nature and structure of their relativistic outflows. These observations will provide new insight into the current debate about the nature of the relativistic ejecta of compact-object mergers and their (now questioned) capability to successfully launch ultra-relativistic jets in their environment. This proposal opens a new window of investigation on GW sources.

PROPOSAL: 10103     PI: Yuhan Yao

PROBING BLACK HOLE SUPER-EDDINGTON ACCRETION IN TIDAL DISRUPTION EVENTS

    Tidal disruption events (TDEs) offer great opportunities to study the physics of black hole accretion in the super-Eddington regime. A growing number of X-ray bright TDEs are being discovered by current and upcoming time domain surveys. We propose two epochs of 50ks NuSTAR + 15ks Swift ToO observations on TDEs with bright hard X-ray emission. For jetted TDEs, this program will shed light on the location, composition and radiation mechanism of relativistic jets launched from super-Eddington accreting black holes. For non-jetted TDEs with non-thermal X-ray emission, this program will provide a broadband X-ray spectrum with relativistic reflection features that allows us to measure key physical quantities of the accretion flow (e.g., disk ionization state, BH spin, inclination).

PROPOSAL: 10112     PI: Qi Feng

SEARCHING FOR NEUTRINO-EMITTING BLAZARS IN HARD X-RAY BAND

    We propose NuSTAR target-of-opportunity (ToO) observations of a candidate neutrino-emitting blazar, triggered by the combination of an IceCube neutrino alert and detections of a spatially coincident blazar by Fermi-LAT and Swift-XRT. We request 40 ks of initial NuSTAR observation within 24 hours, on a best-effort basis, after the trigger. If the hard-X-ray flux from the initial observation is > 1.5 x 10^-12 erg cm^-2 s^-1, we request another 40 ks of follow-up NuSTAR observations to characterize the low-flux state.

PROPOSAL: 10134     PI: Riccardo Middei

NUSTAR AND MULTI-ENERGY SPECTRAL AND POLARIMETRIC OBSERVATIONS OF A FLARING HSP BLAZAR

    We propose a ToO observation with NuSTAR (40 ks) of a high synchrotron peaked (HSP) blazar during a TeV flare. The requested ToO observation lies in the context of an already approved IXPE ToO that is complemented by multiwavelength observations in radio, optical, TeV-gamma-rays. Precise estimates of the photon index alpha and the broadband spectral curvature beta are key ingredients of a robust spectropolarimetric analysis in the X-ray band. Nustar, thanks to its unrivaled bandpass in best suited to accurate model the curved spectrum of an HSP and to track the spectral variability down to few ks timescale. The proposed ToO will provide insight into the acceleration and cooling processes experienced by the particles during the flare.

PROPOSAL: 10171     PI: Jiachen Jiang

A NUSTAR AND NICER VIEW OF THE GALACTIC ULTRALUMINOUS X-RAY PULSAR SWIFT J0243.6+6124

    The detections of pulsations in ultraluminous pulsars suggest that the accreting objects are neutron stars. This puts a robust upper limit on the mass of the compact objects and indicates luminosities that far exceed the Eddington. We request three NuSTAR ToO observations with 20 ks exposure to observe the next outburst of the only Galactic ultraluminous X-ray pulsar Swift J0243.6+6124. Each observation will be implemented by one 3 ks NICER observation to cover the soft X-ray band. These observations will enable us to study the geometry of the innermost accretion region and the magnetic field's strength in this pulsar. In particular, we will consider a super-Eddington disk reflection spectral model based on GRMHD simulations.

PROPOSAL: 10182     PI: Dheeraj Pasham

NUSTAR AND NICER OBSERVATIONS TO IDENTIFY AND STUDY COSMOLOGICAL BLACK HOLES AS THEY TURN ON A RELATIVISTIC JET

    Following NICER+NuSTAR's recent success in capturing the spectro-timing variability of the farthest stellar tidal disruption event (TDE) to-date we propose ToO monitoring observations of a future relativistic TDE, i.e., a system with a newborn relativistic jet pointed directly along our line of sight. Our main goals are 1) to establish the relativistic nature of the future transient by measuring its luminosity and variability, and 2) combine NICER+NuSTAR data with our approved multi-frequency radio data to perform multi-epoch spectral energy distribution modelings to shed light on the underlying jet physics. Our scientific goals require high-cadence monitoring for weeks and hard X-ray coverage making NICER + NuSTAR ideal facilities for the proposed study.

PROPOSAL: 10185     PI: Kirill Sokolovsky

TRACING SHOCK DEVELOPMENT IN A NOVA

    We propose 3x50ks observations of a new bright (V<7.5) nova likely to be detected in gamma-rays. The observations will trace the development of shocks within the nova ejecta, constrain the non-thermal particle acceleration and gamma-ray production mechanisms. We'll put an upper limit on the particle acceleration efficiency by comparing thermal X-ray and optical to GeV luminosity and search for predicted non-thermal X-rays. Understanding nova shocks is relevant for other shock-powered transients: interacting supernovae, tidal disruption events, stellar mergers. NuSTAR is the only instrument capable of detecting hard X-rays from novae simultaneously with the GeV emission. The observations should be conducted now to take advantage of the simultaneous operations with Fermi, Swift and XMM.

PROPOSAL: 10191     PI: Mirko Krumpe

TRACKING THE X-RAY EVOLUTION OF AN ONGOING SDSS-V CHANGING-LOOK AGN

    SDSS-V, now in full operation, is obtaining repeated optical spectroscopy for thousands of AGN. This will allow for the optical identification of extremely rare, currently transitioning changing-look AGN. We request 2 ToOs (each consisting of a 150 ks observation) to track the X-ray spectral evolution of an ongoing optical transition with a cadence of two months. The combined X-ray and optical datasets will explore how the X-ray corona, accretion disc, and broad-emission line region in AGN interact with each other. It will deliver valuable insights into how accretion flows evolve during AGN ignitions and depletions. We will compare our results with the outcomes of a program on X-ray-selected changing-look AGN.

PROPOSAL: 10198     PI: Andrea Gokus

UNCOVERING THE HIGH-ENERGY PROPERTIES OF HIGH REDSHIFT BLAZARS

    We propose a 100ks ToO observation of a high-redshift (z>=3) blazar during a gamma-ray flare detected with Fermi/LAT. Gamma-ray emission has only been detected for 12 high-z blazars so far, mostly while these sources were in outburst. In order to expand the elusive and small sample of gamma-detected MeV blazars, we want to gain simultaneous multi-wavelength data during a flare, that is, when the gamma-ray spectrum can be constrained well for accurate SED modelling. NuSTAR plays a crucial role in our campaign as the hard X-ray data is needed to constrain the high-energy hump of the SED. Our multiwavelength data set will yield insights into the processes of accreting super massive black holes in the early Universe, which are also key targets for future MeV telescopes.

PROPOSAL: 10216     PI: Javier Garcia

A HARD LOOK OR HARDLY LOOKING? A MONITORING CAMPAIGN FOR BRIGHT BLACK HOLE BINARIES

    Transient black hole binaries (BHB) undergo X-ray outbursts that display a plethora of physical phenomena on human timescales, typically lasting several months. This includes rapid changes in both the spectral and timing data of these systems. We request a total of 500 ks of NuSTAR time to monitor any known or new BHB system that goes into an outburst during Cycle 10, triggering ~18 observations to sample all accretion states. We also request 50 ks of quasi-simultaneous NICER time to support this campaign by covering the soft energy band and providing fast timing observations. The key goal is to provide a comprehensive view of the properties of accreting BHs, including robust measurements of their mass and spin via spectral-timing modeling of the X-ray data.

PROPOSAL: 10220     PI: Megumi Shidatsu

UNDERSTANDING DISK WINDS IN PREVIOUSLY UNCOVERED STATES OF BLACK HOLE X-RAY BINARIES

    We propose NuSTAR observations of transient BHXBs simultaneously with the XRISM ToO observations, to study the properties of disk winds in the low/hard state and in low inclination angle sources, which are not well understood. The new satellite XRISM is expected to revolutionize these studies, with unprecedented weak lines detection and resolution capabilities. Simultaneous NuSTAR observations enable us to determine the shape of the hard X-ray continuum spectrum, which is crucial to determine wind properties from the XRISM high resolution spectrum. Moreover, NuSTAR can provide the reflection features. Combining them with the information of narrow absorption lines obtained with XRISM, we can reveal the underlying wind mechanisms and the accretion disk structure in the low/hard state.

PROPOSAL: 10245     PI: Guglielmo Mastroserio

SPECTRAL-POLARIMETRIC ANALYSIS DURING THE OUTBURST OF GX 339-4

    X-ray polarization has proven to be effective in constraining the orientation of the innermost regions of black hole binaries (BHBs). However, the IXPE telescope covers a very small energy range of 2-8 keV which does not allow us to properly distinguish among the different spectral components. It is crucial to combine the new polarimetric diagnostics with a proper separation of the different spectral components to relate each component with its polarisation measurements. NuSTAR observations are critical to perform this task. Thus, we propose to observe GX 339-4 with NuSTAR during the 3 IXPE exposures already scheduled for the hard, intermediate, and soft states. We request the minimum NuSTAR exposure 20~ks for each observation for a total of 60~ks which has already proven to be effective.

PROPOSAL: 10246     PI: Daniel Lawther

MAPPING OUT THE X-RAY SOURCE IN A CHANGING-LOOK AGN

    After a decade of quiescence, the changing-look AGN Mrk 590 has been in a repeat flaring state since 2017. Its strong, coherent and delayed UV response to X-rays is inconsistent with standard models of the AGN central engine. Here, we propose deep NuSTAR and XMM-Newton observations to capture Mrk 590 during the peak of an X-ray flare. These observations will test whether hitherto undetected signatures of a thin accretion disk appear as the flares reach their highest luminosities. They also provide timing information with which we will measure the distances between the primary X-ray continuum (hot corona), soft X-ray excess, and reflection component. This will probe whether the primary X-ray continuum is emitted co-spatially with the warm Comptonization and UV-emitting regions.

PROPOSAL: 10249     PI: Sean Pike

RAPID FOLLOW-UP OF UNKNOWN MAXI SOURCES NEAR THE SUN WITH NUSTAR

    We propose rapid follow-up observations of up to 3 unknown MAXI X-ray transients which lie between 10 and 47 degrees of the Sun using NuSTAR in order to tile the corresponding MAXI error region, localize the source, and perform spectral analyses for the purpose of source classification. Sources near the Sun often go unobserved due to the limitations of many X-ray observatories. The ToO program we propose will ensure that observations are carried out for sources that would be unobservable to observatories other than NuSTAR. As demonstrated by a number of recent NuSTAR observations at low Sun angle, observations like this have the potential to help answer important astrophysical questions regarding the evolution of accretion disks throughout outbursts, the onset of jet formation, and more.

PROPOSAL: 10251     PI: Peter Kosec

PROBING THE RESPONSE OF AN ACCRETION DISK WIND TO X-RAY PULSATIONS

    Her X-1 is a pulsar with a period of 1.24 s and an ionized accretion disk wind. We will leverage the ionization response of the wind to the X-ray pulsation to constrain the wind number density, in turn constraining its launching mechanism. We will combine the high-resolution Chandra gratings (50 ks exposure already approved in the previous proposal cycle, to be carried out in 2024) with the broad 3-79 keV spectral coverage of NuSTAR. NuSTAR is crucial to accurately track the time-variable broadband spectral energy distribution of Her X-1, allowing us to measure the wind ionizing balance at all times and thus its ionization state. We request two 20-ks NuSTAR observations to be carried out simultaneously with our Chandra exposure (likely to be split into two ~25-ks observations).

PROPOSAL: 10257     PI: Amruta Jaodand

CHASING THE X-RAY AFTERGLOWS OF GRAVITATIONAL-WAVE EVENTS

    Many astrophysical questions regarding NS mergers still remain open. What fraction of them have central engines and how long do they operate? What is the maximum mass for a stable NS remnant? How much energy do mergers release? Do all mergers produce successful relativistic jets and short GRBs? Broadband X-ray observations (Using soft and hard X-ray observations) of diverse NS merger events (especially in conjunction with radio observations) will be able to answer many of these questions. In LIGO O4 we expect about 2--7 GW events with electromagnetic counterparts, and therefore arc-second localizations. NuSTAR's broadband coverage and ability to look close-to/at the Sun will crucial for following-up these neutron star gravitational wave mergers.

PROPOSAL: 10267     PI: Jon Miller

TRIGGERED NUSTAR OBSERVATIONS OF SEYFERTS IN FAST OUTFLOW STATES

    The combination of NuSTAR and XRISM has the potential to open new windows on ultra-fast outflows and feedback in AGN. NuSTAR is uniquely able to define the hard continuum that ionizes the Fe XXV and Fe XXVI lines that define most UFOs, while XRISM can reveal their velocity structure, driving mechanisms, and kinetic power. However, UFOs are known to be variable, and it is vital to observe the right sources at the right times. We propose to use hardness-intensity monitoring to trigger NuSTAR and XRISM observations of PG 1211+143 and Mrk 817. These sources optimize a combination of source flux and UFO strength, and may launch UFOs via distinct mechanisms. We request an 80 ks observation of PG 1211+143 and 100 ks observation of Mrk 817 during NuSTAR Cycle 10.

PROPOSAL: 10268     PI: Benjamin Coughenour

SPIN AND REFLECTION IN A BLACK HOLE TRANSIENT

    We propose a joint 50 ks NuSTAR and 5 ks NICER ToO observation of a transient black hole (BH) X-ray binary during outburst. Using up to an additional 5 ks with NICER to monitor the source state, we will trigger the primary joint NuSTAR/NICER observation when the source transitions from the hard state to its intermediate state. The BH spin will be measured by modeling reflection signatures in the source spectrum, which will also constrain properties of the innermost accretion environment. Targets include sources that have not yet been observed by NuSTAR during outburst, whether that means an entirely new transient BH candidate or a previously known BH X-ray binary that has been in quiescence since the launch of NuSTAR (GRO J1655-40 or XTE J1550-564, e.g.).

PROPOSAL: 10309     PI: Oluwashina Adegoke

PROBING OBSCURATION, DISK WINDS AND THEIR RELATIONSHIP WITH ACCRETION STATES IN BLACKHOLE X-RAY BINARIES

    A few blackhole X-ray binaries (BHXRBs) show recurrent intensity dips in their light curves during outburst. These systems also always show indication of disk winds. The dips are believed to be caused by clumps of obscuring material passing along the line of sight to the X-ray source. Our knowledge of the origin of such obscurer, its composition and any relation with disk winds as well as spectral state is limited. Because soft X-ray photons are mostly absorbed by the obscuring material, NuSTAR's high energy capabilities make it the instrument of choice to study the nature of dips and obscuration in BHXRBs. NICER s high S/N also allows to probe the evolution of these features. We Therefore propose target of Opportunity observations with NuSTAR jointly with NICER to study these features.

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Joint NuSTAR / Chandra

PROPOSAL: 17400584     PI: Mark Reynolds

Accretion disk winds in the presence of a neutron star with a 1e12 G magnetic field

    We propose to obtain a 30 ks Chandra/HETGS and quasi-simultaneous 20 ks NuSTAR ToO observation of the next bright outburst from the Be/X-ray binary 1A0535+262. We will trigger the proposed observation when the source is at a luminosity in excess of 10% Eddington to further constrain the high velocity wind in this source and to probe accretion, at large fractions of the Eddington luminosity, onto the accreting pulsar with the largest known magnetic field (4e12 G). The NuSTAR high energy data will provide constraints on the broadband spectrum, and will accurately constrain the cyclotron line energy at the highest luminosity observed to date and place constraints on the geometry of the accretion column onto the neutron star at high Eddington fractions.

PROPOSAL: 18500288     PI: Nanda Rea

Prompt study of magnetar outbursts with Chandra and NuSTAR

    The discovery of transient magnetars has opened a new perspective in the field confirming that a relatively large number of members of this class has not been discovered yet, and suggesting that others would manifest themselves in the future through outbursts. This proposal is aimed at gathering new insights on the physics of magnetars through the study of their outbursts. In particular, we are asking for 85ks of Chandra time divided in 4 observations throughout the outburst from a known or still unknown magnetar candidate, as well as 85ks of NuSTAR time to monitor the hard X-ray emission during the outburst decay.

PROPOSAL: 18500371     PI: Victoria Kaspi

Target-of-oppurtunity chandra observations of magnetars in outburst

    Magnetars are highly magnetized young neutron stars that are powered by the decay of their enormous magnetic fields. Magnetars exhibit dramatic variability including major outbursts _ a hallmark of their emission. These outbursts allow us to probe the extreme physics at work in the UniverseÍs largest known magnetic fields. Understanding the evolution of these outbursts will place stringent constraints on physical models of magnetars, including their crusts, cores, atmospheres, coronae, and magnetospheres. In this proposal, we request Chandra and NuSTAR Target-of-Opportunity observations of a major magnetar outburst in AO18 in order to put detailed physical models to the test, and as an anchor for multiwavelength follow-up.

PROPOSAL: 18500379     PI: Raffaell Margutti

Mapping extreme mass loss from evolved massive stars with coordinated Chandra-NuSTAR observations

    Contrary to expectations from current stellar evolutionary models, recent observations uncovered the ejection of shells of material by massive stars in the years before the supernova (SN) explosion. The physical mechanism behind the impulsive mass ejection synchronized with the stellar core-collapse is unclear. Here we propose a coordinated Chandra-NuSTAR effort to map the evolution of the broad-band X-ray spectrum of 1 nearby (d<50 Mpc) strongly interacting SN and enable progress. Our program is designed to characterize the medium around strongly interacting SN, which originate from stellar progenitors with the most extreme mass loss before explosion. The overarching goal is to constrain the nature of the physical process responsible for mass loss in evolved massive stars.

PROPOSAL: 19400282     PI: J Neilsen

An Integrated Approach to Winds/ Jets/ and State Transitions

    We propose a large multiwavelength campaign (120 ks Chandra HETGS, NuSTAR, INTEGRAL, JVLA/ATCA, Swift, XMM, Gemini) on a black hole transient to study the influence of ionized winds on relativistic jets and state transitions. With a reimagined observing strategy based on new results on integrated RMS variability and a decade of radio/X-ray monitoring, we will search for winds during and after the state transition to test their influence on and track their coevolution with the disk and the jet over the next 2-3 months. Our spectral and timing constraints will provide precise probes of the accretion geometry and accretion/ejection physics.

PROPOSAL: 19400584     PI: J F Steiner

A Test of Black-Hole Disk Truncation: Thermal Disk Emission in the Bright Hard State

    The assumption that a black holes accretion disk extends inwards to the ISCO is on firm footing for soft spectral states, but has been challenged for hard spectral states where it is often argued that the accretion flow is truncated far from the horizon. This is of critical importance because black-hole spin is measured on the basis of this assumption. The direct detection (or absence) of thermal disk emission associated with a disk extending to the ISCO is the smoking-gun test to rule truncation in or out for the bright hard state. Using a self-consistent spectral model on data taken in the bright hard state while taking advantage of the complementary coverage and capabilities of Chandra and NuSTAR, we will achieve a definitive test of the truncation paradigm.

PROPOSAL: 20400577     PI: J Neilsen

An Integrated Approach to Winds, Jets, and State Transitions

    We propose a large multiwavelength campaign (120 ks Chandra HETGS, NuSTAR, JVLA/ATCA, NICER, Swift, Gemini) on a black hole transient to study the influence of ionized winds on relativistic jets and state transitions. With a reimagined observing strategy based on new results on integrated RMS variability and a decade of radio/X-ray monitoring, we will search for winds during and after the state transition to test their influence on and track their coevolution with the disk and the jet over the next 2-3 months. Our spectral and timing constraints will provide precise probes of the accretion geometry and accretion/ejection physics.

PROPOSAL: 20400601     PI: J F Steiner

A Test of Black-Hole Disk Truncation: Thermal Disk Emission in the Bright Hard State

    The assumption that a black hole's accretion disk extends inwards to the ISCO is on firm footing for soft spectral states, but has been challenged for hard spectral states where it is often argued that the accretion flow is truncated far from the horizon. This is of critical importance because black-hole spin is measured on the basis of this assumption. The direct detection (or absence) of thermal disk emission associated with a disk extending to the ISCO is the smoking-gun test to rule truncation in or out for the bright hard state. Using a self-consistent spectral model on data taken in the bright hard state while taking advantage of the complementary coverage and capabilities of Chandra and NuSTAR, we will achieve a definitive test of the truncation paradigm.

PROPOSAL: 21400516     PI: K Mori

Follow-up TOO Chandra and NuSTAR observations of Swift X-ray transients in the Galactic Center

    We propose Chandra and NuSTAR follow-up observations of new X-ray transients and recurrent outbursts from the known VFXTs (very faint X-ray transients), detected by Swift-XRT, at r < 50 pc from the Galactic Center. The accurate source localization by Chandra and broad-band spectral/timing data with NuSTAR provide the useful diagnostic tools of understanding the nature of X-ray transients in the Galactic Center, as demonstrated by four Swift transients followed up by Chandra and NuSTAR including the transient magnetar SGR J1745-2900, one VFXT and two new X-ray transients. Detecting and characterizing new X-ray transients, together with the quiescent X-ray binaries recently discovered by Chandra (Hailey et al. 2018) will probe the population and formation of X-ray binaries near Sgr A*.

PROPOSAL: 22400400     PI: K Mori

Follow-up TOO Chandra and NuSTAR observations of Swift X-ray transients in the Galactic Center

    We propose Chandra and NuSTAR follow-up observations of new X-ray transients and recurrent outbursts from the known VFXTs (very faint X-ray transients), detected by Swift-XRT, at r < 50 pc from the Galactic Center. The accurate source localization by Chandra and broad-band spectral/timing data with NuSTAR provide the useful diagnostic tools of understanding the nature of X-ray transients in the Galactic Center, as demonstrated by four Swift transients followed up by Chandra and NuSTAR including the transient magnetar SGR J1745-2900, one VFXT and two new X-ray transients. Detecting and characterizing new X-ray transients, together with the quiescent X-ray binaries recently discovered by Chandra (Hailey et al. 2018) will probe the population and formation of X-ray binaries near Sgr A*.

PROPOSAL: 22400529     PI: M Bachetti

Timing the M82 X-2 pulsar with Chandra and NuSTAR

    The galaxy M82 harbors two notable ultraluminous X-ray sources (ULX): M82 X-2, the archetipal pulsating ultraluminous X-ray source (PULX), and M82 X-1, a IMBH candidate. The extreme luminosity of ULXs, and the observation of strong winds from these sources, suggests that these sources are undergoing strongly super-Eddington accretion. The supply of matter that powers these accreting systems is likely to come from Roche Lobe overflow from a more massive companion star, and should produce changes of orbital period observable in time scales of ~years. We have recently detected such orbital period derivative thanks to NuSTAR. This program aims at following up on this orbital period derivative through pulsar timing in M82 X-2, using Chandra to select the ideal observing times.

PROPOSAL: 22500172     PI: P Scholz

Simultaneous Chandra, NuSTAR and Radio Observations of CHIME-discovered repeating FRBs

    Fast Radio Bursts (FRBs) are a new class of astrophysical transient of unknown origins. The CHIME/FRB project is now operational and discovering several FRBs per day. A fraction of the sources are repeating and can potentially be localized to arcsecond precision using radio interferometers. These precise localizations will enable a deep search for X-ray counterparts of the radio bursts. Some models, notably those involving magnetars, predict X-ray bursts along with FRBs which may be detectable with Chandra or NuSTART for nearby sources. We therefore propose for 54 ks of Chandra ToO observations to be scheduled along with 15 hr of Effelsberg and 27 ks of NuSTAR observations on a well-localized CHIME/FRB repeater.

PROPOSAL: 22500192     PI: D Matthews

Chandra-NuSTAR Observations of Rapid, Luminous, and Blue Stellar Explosions

    We propose an in-depth X-ray study of fast blue optical transients (FBOTs), a new class of transients with luminosities and time scales that challenge traditional SN models. Alternative scenarios include a failed SN of a stripped star, He-shell detonation on a white dwarf, and a SN shock breaking through a dense medium. FBOTs have mostly been studied in the optical/UV regime, which is of thermal origin and it is not sensitive to the nature of the underlying energy source. Here we propose to capitalize on our recent discovery of X-rays with unprecedented properties from the FBOTs AT2018cow and CSS161010, and start the first broad-band X-ray monitoring campaign of FBOTs. The primary goal is to test for the presence of engines driving the explosions and constrain their nature.

PROPOSAL: 23400247     PI: Neilsen

Searching for Winds and Jets in Black Hole State Transitions

   We propose a large multiwavelength campaign (120 ks Chandra HETGS, NuSTAR, JVLA/ATCA, NICER, Swift, Gemini) on a black hole transient to study the influence of ionized winds on relativistic jets and state transitions. With a new observing strategy based on integrating radio/X-ray monitoring and timing studies, we will search for winds during and after the state transition to test their influence on and track their coevolution with the disk and the jet over the next 2-3 months. Our spectral and timing constraints will provide precise probes of the accretion geometry and accretion/ejection physics.

PROPOSAL: 23500541     PI: Chornock

A broad-band view of Extreme Mass Loss events from Evolved Massive Stars with Chandra+NuSTAR

   Contrary to expectations from stellar evolutionary models, recent observations uncovered the ejection of shells of material massive stars in the years before the supernova (SN) explosion. The physical mechanism behind the impulsive mass ejection synchronized with the stellar core-collapse is unclear. Here we propose a coordinated Chandra-NuSTAR effort to map the evolution of the broad-band X-ray spectrum of 1 nearby (d<50 Mpc) strongly interacting SN and enable progress. Our program is designed to characterize the medium around strongly interacting SN, which originate from stellar progenitors with the most extreme mass loss before explosion. The overarching goal is to constrain the nature of the physical process responsible for mass loss in evolved massive stars.

PROPOSAL: 24400072     PI: Mori

Chandra and NuSTAR ToO observations of Swift X-ray transients in the Galactic Center

   We propose Chandra and NuSTAR follow-up observations of new X-ray transients and recurrent outbursts from the known VFXTs (very faint X-ray transients), detected by Swift-XRT, at r < 50 pc from the Galactic Center. The accurate source localization by Chandra and broad-band spectral/timing data with NuSTAR provide the useful diagnostic tools of understanding the nature of X-ray transients in the Galactic Center, as demonstrated by four Swift transients followed up by Chandra and NuSTAR including the transient magnetar SGR J1745-2900, one VFXT and two new black hole transients. Detecting and characterizing new X-ray transients, together with investigating quiescent X-ray binaries, will probe the population and formation of X-ray binaries near Sgr A*.

PROPOSAL: 24400242     PI: Neilsen

Searching for Winds and Jets in Black Hole State Transitions

   We propose a large multiwavelength campaign (120 ks Chandra HETGS, NuSTAR, JVLA/ATCA, NICER, Swift, Gemini) on a black hole transient to study the influence of ionized winds on relativistic jets and state transitions. With a new observing strategy based on integrating radio/X-ray monitoring and timing studies, we will search for winds during and after the state transition to test their influence on and track their coevolution with the disk and the jet over the next 2-3 months. Our spectral and timing constraints will provide precise probes of the accretion geometry and accretion/ejection physics.

PROPOSAL: 24500203     PI: Brethauer

Mapping extreme mass loss from evolved massive stars with coordinated Chandra-NuSTAR observations

   Contrary to expectations from stellar evolutionary models, recent observations uncovered the ejection of shells of material massive stars in the years before the supernova (SN) explosion. The physical mechanism behind the impulsive mass ejection synchronized with the stellar core-collapse is unclear. Here we propose a continuation of our coordinated Chandra-NuSTAR effort to map the evolution of the broad-band X-ray spectrum of 1 nearby (d<50 Mpc) strongly interacting SN and enable progress. Our program is designed to characterize the medium around strongly interacting SN, which originate from stellar progenitors with the most extreme mass loss before explosion. The overarching goal is to constrain the nature of the physical process responsible for mass loss in evolved massive stars.

PROPOSAL: 24500280     PI: Matthews

Mining the Transient Sky: Chandra-NuSTAR Observations of the Fastest Explosions

   We propose an in-depth X-ray study of fast blue optical transients (FBOTs), a new class of transients with luminosities and time scales that challenge traditional SN models. Alternative scenarios include a failed SN of a stripped star, He-shell detonation on a white dwarf, and a SN shock breaking through a dense medium. FBOTs have mostly been studied in the optical/UV regime, which is of thermal origin and it is not sensitive to the nature of the underlying energy source. Here we propose to capitalize on the recent discovery of X-rays from the FBOT AT2018cow with unprecedented properties, and start the first broad-band X-ray monitoring campaign of FBOTs. The primary goal is to test for the presence of engines driving the explosions and constrain their nature.

PROPOSAL: 25400276     PI: Kosec

Resolving Powerful Outflows in a Nearby Transient Ultraluminous X-ray Source

   In recent years, high resolution spectroscopy revealed the presence of radiatively driven ultrafast outflows in super-Eddington ultraluminous X-ray sources. Unfortunately, because most ULXs are at Mpc distances, the low signal-to-noise of these spectra limit our ability to probe the plasma properties and their evolution in time. Here we present a unique opportunity to obtain the highest quality high-resolution spectra of a ULX by targeting a transient ULX in the Magellanic Clouds. The quality of these three 25 ks ToO observations (joint with NuSTAR) will rival in statistics the future ATHENA spectra of bona fide ULXs. These observations will easily detect fast outflows in absorption (or place deep limits on their presence), as well as allow us to study the properties of the ionized plasma in emission, commonly seen in regular ULXs. We will accurately measure the ionized outflow properties, estimate its energetics and study both its short-term and long-term evolution with key ULX parameters.

PROPOSAL: 25400338     PI: Neilsen

Searching for Winds and Jets in Black Hole State Transitions

   We propose a large multiwavelength campaign (120 ks Chandra HETGS, NuSTAR, JVLA/ATCA, NICER, Swift, Gemini) on a black hole transient to study the influence of ionized winds on relativistic jets and state transitions. With a new observing strategy based on integrating radio/X-ray monitoring and timing studies, we will search for winds during and after the state transition to test their influence on and track their coevolution with the disk and the jet over the next 2-3 months. Our spectral and timing constraints will provide precise probes of the accretion geometry and accretion/ejection physics.

PROPOSAL: 26400066     PI: Peter Kosec

Resolving Powerful Outflows in a Cornerstone Transient Ultraluminous X-ray Source

   Recently, high resolution spectroscopy revealed radiatively-driven ultrafast outflows in super-Eddington Ultraluminous X-ray Sources. Because most ULXs are at Mpc distances, the low signal-to-noise of these spectra limit our ability to probe the plasma properties and their evolution in time. We present a unique opportunity to obtain the highest quality high-resolution ULX spectra by targeting a transient ULX in the Magellanic Clouds. The quality of these three 25 ks ToO observations (joint with NuSTAR) will rival in statistics the future ATHENA spectra of bona fide ULXs. These observations will easily detect fast outflows in absorption (or place deep limits on their presence), as well as allow us to study the properties of the ionized plasma in emission, commonly seen in regular ULXs.

PROPOSAL: 26400115     PI: Kaya Mori

Chandra and NuSTAR observations of X-ray transients in the Galactic Center

   We propose Chandra and NuSTAR follow-up observations of new X-ray transients and recurrent outbursts from the known VFXTs (very faint X-ray transients), detected by Swift-XRT at r < 50 pc from the Galactic Center. The accurate source localization by Chandra and broad-band spectral/timing data with NuSTAR provide useful diagnostic tools for understanding the nature of X-ray transients in the Galactic Center, as demonstrated by four Swift transients followed up by Chandra and NuSTAR including the transient magnetar SGR J1745-2900, one VFXT and two new black hole transients. Detecting and characterizing new X-ray transients, together with investigating quiescent X-ray binaries, will probe the population and formation of X-ray binaries near Sgr A*.

PROPOSAL: 26400240     PI: Joey Nielsen

Searching for Winds and Jets in Black Hole State Transitions

   We propose a large multiwavelength campaign (120 ks Chandra HETGS, NuSTAR, JVLA/ATCA, NICER, Swift, Gemini) on a black hole transient to study the influence of ionized winds on relativistic jets and state transitions. With a new observing strategy based on integrating radio/X-ray monitoring and timing studies, we will search for winds during and after the state transition to test their influence on and track their coevolution with the disk and the jet over the next 2-3 months. Our spectral and timing constraints will provide precise probes of the accretion geometry and accretion/ejection physics.

PROPOSAL: 26500151     PI: Raffaella Margutti

Chandra and NuSTAR observations of the most extreme mass-loss events from massive stars

   Observations in the last decade have revealed a complex mass-loss history of massive stars on the verge of core-collapse. The physical mechanism behind the impulsive mass ejections synchronized with the stellar core-collapse is currently unknown, with theoretical proposals ranging from nuclear burning instabilities to binary interactions. We propose a broad-band coordinated effort with Chandra and NuSTAR to constrain the evolution of a strongly interacting SN in the local Universe (d<50 Mpc) or X-ray bright, and enable progress. Our program is designed to characterize the medium around strongly interacting SNe, which originate from massive stars with the most extreme mass loss before explosion. The overarching goal is to constrain the nature of the underlying physical process(es).

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Joint NuSTAR / XMM-Newton

PROPOSAL: 074462     PI: Sandro Mereghetti

Measuring the strongest magnetic fields in the universe

    AXPs and SGRs are believed to be magnetars, ultra-magnetised neutron stars mainly powered by huge magnetic fields. We recently discovered a phase-dependent absorption line in the X-ray spectrum of SGR 0418+5729, which implies the existence of localized magnetic structures with B~10^15 G, despite its much lower dipolar field, as predicted by the magnetar model. We propose to use XMM-Newton and NuSTAR to study in more detail the spectral line in this source (if it undergoes a new outburst) or any other similar feature discovered in other magnetar candidates.

PROPOSAL: 076203     PI: Victoria Kaspi

Target-of-opportunity XMM and NuSTAR observations of magnetars in outburst

    Magnetars are highly magnetized young neutron stars that are powered by the decay of their enormous magnetic fields. Magnetars exhibit dramatic variability including major outbursts -- a hallmark of their emission. These outbursts allow us to probe the extreme physics at works in the Universe's largest known magnetic fields. Understanding the evolution of these outbursts will place stringent constraints on physical models of magnetars, including their crusts, atmospheres, coronae, and magnetospheres. In this proposal, we request XMM EPIC and NuSTAR Target-of-Opportunity observations of a major magnetar outburst in AO-14 in order to constrain the detailed physics of magnetars.

PROPOSAL: 076434     PI: Alessandro Papitto

Hunting for transitional millisecond pulsars with XMM-Newton and NuSTAR

    The XMM discovery of a ms pulsar swinging between an accretion-powered (X-ray) and a rotation-powered (radio) pulsar state demonstrated that transitions between the two states can be observed over timescales of a few weeks. We propose an XMM/NuStar ToO observation of 60 ks aimed at studying the accretion state of transitional ms pulsars, detecting X-ray accretion powered pulsations, and characterizing its variability over three decades in energy. Candidates are restricted to black widows and redbacks, systems in an evolutionary phase that allows state transitions. Enlarging the number of systems in this transitional phase is crucial to test binary evolution theories, and to study the disk-field interaction over a large range of mass accretion rates.

PROPOSAL: 078050     PI: Norbert Schartel

Catching AGN in deep minimum states to unveil their core environment

    The deep minimum state of AGNs is characterized by a strongly suppressed or even absent primary continuum. As the continuum disappears weak spectral features like relativistic iron lines or narrow soft X-ray emission lines from ionized plasmas become highly significant and their parameters can be determined. Therefore deep minimum states offer unique possibilities to investigate in detail the physics of the reprocessed components in AGN, including the immediate vicinity of the supermassive black hole. Applying our experience (several deep minimum observations) we propose 2 triggered 10ks XMM snapshot, one 80ks XMM follow-up simultaneous with one 80ks NuSTAR one 2-orbit HST observation of an AGN in deep minimum state. We will identify deep minimum states mainly based on Swift and XMM-Newton slews.

PROPOSAL: 078214     PI: Felix Fuerst

The accretion geometry in a black hole binary at very low luminosities

    The combination of XMM and NuSTAR for broad-band X-ray spectroscopy is one of the most effective means today of exploring accretion in Galactic black holes. Due to the high quality spectra, reflection from the inner accretion disk and relativistic effects can be studied with unprecedented detail. Here we propose to make a joint 80ks XMM-Newton plus 40ks NuSTAR observation of a new or known black hole transient during the end of an outburst, after it enters the low-hard state. This proposal is a continuation of our AO 14 program (which we triggered on GRS 1739 278 in 2015 September) extending it to other sources and luminosity levels. Our goals are to measure the inner truncation radius of the accretion disk and to study the accretion geometry before the black hole returns to quiescence.

PROPOSAL: 078354     PI: Adam Ingram

Iron line doppler tomography with quasi-periodic oscillations

    We propose three joint XMM-Newton and NuSTAR observations of the black hole binary H 1743-322, each covering a single XMM-Newton orbit and 70ks of NuSTAR time, sampling different stages of spectral evolution during the rise to outburst. This builds upon our recent discovery that the iron line centroid energy in this source varies systematically over the course of a 4s QPO cycle, providing strong evidence that the QPO is driven by relativistic precession. We now aim to sample the same source when displaying a range of higher QPO frequencies to explore the evolution of the iron line profile phase dependence with QPO frequency and compare with the prediction of relativistic precession. We will trigger the observations from Swift monitoring throughout the outburst.

PROPOSAL: 078430     PI: Nanda Rea

Prompt XMM-Newton and NuSTAR study of magnetar outbursts

    The magnetar candidates (comprising AXPs and SGRS) are a small class of neutron stars mainly powered by their huge magnetic fields. They go through long stretches of quiescence, interrupted by periods of activity, via short X-ray bursts, large giant flares and yearly-timescale X-ray outbursts. Observations performed during, or immediately after such periods yield the largest amount of information on magnetars' emission, allowing one to test the theoretical models on a variety of phenomena and source states. We propose a Joint XMM (85ks) and NuSTAR (170ks) ToO program (1 trigger of 4 observations over a few months) aimed at gathering new physical insight on magnetars crust, field configuration, magnetospheres, and evolution.

PROPOSAL: 080084     PI: Norbert Schartel

Outbursts of radio-quiet AGN

    Recent XMM-Newton observations of Seyfert galaxies establish outbursts of radio-quiet AGNs as a poorly explored discovery space for AGN physics with an enormous potential to learn. We propose two 80ks simultaneous XMM-Newton/NuSTAR observations accompanied by SALT/HET optical spectroscopy of the next suited AGN outburst(s), triggered with XMM-Newton slews, Swift, Gaia and others. True AGN outbursts will allow us to trace accretion physics, X-ray spectral complexity connected with dramatic emission-line changes and possibly elusive stellar tidal disruption events probing an even more extreme accretion regime. With the detection of a new rare "Changing Look AGN" in July 2014 we demonstrated successful search strategy and organization of follow-up campaigns with broad wavelength coverage.

PROPOSAL: 080230     PI: Fiona Harrison

The hard state of a transient black hole: is the accretion disk truncated?

    The combination of the fast EPIC-pn modes and broad-band coverage with RXTE was an effective means of exploring accretion in bright Galactic black holes. XMM-Newton and NuSTAR provide an even more powerful combination. We propose one joint 40 ks observation of a new or known black hole transient during a bright, hard outburst. The goals of this program are to 1) measure the inner accretion disk radius of the black hole via disk reflection spectrum and infer the black hole spin, 2) test an apparent anti-correlation between winds and jets in soft and hard states by making a very sensitive search for winds in a hard state. This observation will be supported by a network of ground-based observatories and X-ray monitoring and is in continuation of our successful program in AO 13 15.

PROPOSAL: 080333     PI: Gabriele Ponti

Tracing the evolution of BH winds through the intermediate states

    Black hole transients (BHT) are the only class of black holes (BH) which samples a wide range of accretion rates and accretion states during their month to year long outbursts. BHT classically show two types of outflows: collimated radio-jets ubiquitous during the canonical hard state and highly ionised X-ray winds during the soft state. We propose a combined XMM+NuSTAR monitoring campaign of an high inclination BHT, to shed light on the evolution of the wind while the source transits through the intermediate states.

PROPOSAL: 082053     PI: Norbert Schartel

Catching AGN in Deep Minimum States to Unveil Their Core Environment

    The deep minimum state of AGNs is characterized by a strongly suppressed or even absent primary continuum. As the continuum disappears weak spectral features like relativistic iron lines or narrow soft X-ray emission lines from ionized plasmas become highly significant and their parameters can be determined. Therefore deep minimum states offer unique possibilities to investigate in detail the physics of the reprocessed components in AGN, including the immediate vicinity of the supermassive black hole. Applying our experience (several deep minimum observations) we propose 2 triggered 10ks XMM snapshot, one 80ks XMM follow-up simultaneous with one 80ks NuSTAR one 2-orbit HST observation of an AGN in deep minimum state. We will identify deep minimum states mainly based on Swift and XMM-Newton slews.

PROPOSAL: 082145     PI: Alessandro Papitto

Hunting for transitional millisecond pulsars with XMM-Newton and NuStar

    The XMM discovery of a ms pulsar swinging between an accretion-powered (X-ray) and a rotation-powered (radio) pulsar state demonstrated that transitions between the two states can be observed over timescales of a few weeks. We propose an XMM/NuStar ToO observation of 60 ks aimed at studying the accretion state of transitional ms pulsars, detecting X-ray accretion powered pulsations, and characterizing its variability over three decades in energy. Candidates are restricted to black widows and redbacks, systems in an evolutionary phase that allows state transitions. Enlarging the number of systems in this transitional phase is crucial to test binary evolution theories, and to study the disk-field interaction over a large range of mass accretion rates.

PROPOSAL: 082359     PI: Nanda Rea

Prompt XMM-Newton and NuSTAR study of magnetar outbursts

    Magnetars are a small class of neutron stars mainly powered by their huge magnetic fields. They go through long stretches of quiescence, interrupted by periods of activity, via large flares and yearly-timescale X-ray outbursts. Magnetar activity has been recently observed also in other pulsar classes. Observations performed during, and immediately after such periods yield the largest amount of information on this magnetic emission, allowing one to test the theoretical models on a variety of phenomena and source states. We propose a Joint XMM (85ks) and NuSTAR (170ks) ToO program (1 trigger of 4 observations over a few months) aimed at gathering new physical insight on the crust and field configuration of sources showing magnetar outbursts.

PROPOSAL: 084079     PI: Fiona Harrison

The bright hard state of a black hole binary: probing inner accretion properties

    The combination of fast EPIC-pn modes and broadband coverage with NuSTAR is an effective means of exploring accretion in Galactic black holes. We propose to make one joint 40 ks observation of a new or known black hole transient during a bright, hard outburst phase. The goals of this program are to 1) determine whether the accretion disk is truncated via modeling the disk reflection spectrum, and estimate the black hole spin if no significant truncation is detected 2) study fast variability behaviors, searching for QPOs in the power spectrum and comparing with theoretical models 3) test an apparent anti-correlation between winds and jets in soft and hard states by making a sensitive search for winds. This observation will be supported by ground-based observatories and X-ray monitoring.

PROPOSAL: 084080     PI: Norbert Schartel

Outbursts of radio-quiet AGN

    Recent XMM-Newton observations of Seyfert galaxies establish outbursts of radio-quiet AGNs as a poorly explored discovery space for AGN physics with an enormous potential to learn: GSN 069 is most likely a high Eddington-ratio true Seyfert 2 galaxy with no broad line region and no X-ray continuum. NGC 1566, HE1136-2304 and NGC 2617 are rare changing look AGNs with the latter possibly showing absorption by in-falling matter. We propose two 80ks simultaneous XMMNewton/NuSTAR observations accompanied by optical spectroscopy of the next suited AGN outburst(s). They will allow tracing accretion physics, spectral complexity connected with dramatic emission-line changes and possibly elusive tidal disruption events probing an even more extreme accretion regime.

PROPOSAL: 084355     PI: Raffaella Margutti

XMM-NuSTAR Observations of Rapid, Luminous and Blue Stellar Explosions

    We propose an in-depth X-ray study of fast blue optical transients (FBOTs), a new class of transients with luminosities and time scales that challenge traditional SN models. Alternative scenarios include a failed SN of a stripped star, He-shell detonation on a white dwarf, and a SN shock breaking through a dense medium. FBOTs have mostly been studied in the optical/UV regime, which is of thermal origin and it is not sensitive to the nature of the underlying energy source. Here we propose to capitalize on our recent discovery of X-rays from the FBOT AT2018cow with unprecedented properties, and start the first broad-band X-ray monitoring campaign of FBOTs. The primary goal is to test for the presence of engines driving the explosions and constrain their nature.

PROPOSAL: 084377     PI: Norbert Schartel

Catching AGN in Deep Minimum States to Unveil Their Core Environment

    The deep minimum state of AGN is characterized by a strongly suppressed or even absent primary continuum. As the continuum disappears weak spectral features like relativistic iron lines or narrow soft X-ray emission lines from ionized plasmas become highly significant and their parameters can be determined. Therefore deep minimum states offer unique possibilities to investigate in detail the physics of the reprocessed components in AGN, including the immediate vicinity of the supermassive black hole. Applying our experience (several deep minimum observations) we propose 2 triggered 10ks XMM snapshot, one 80ks XMM follow-up simultaneous with one 80ks NuSTAR one 2-orbit HST observation of an AGN in deep minimum state.

PROPOSAL: 084434     PI: Jelle Kaastra

Shining light on obscuring outflows in AGN

    AGN can be obscured by gas streams close to the black hole that shield remote regions from ionising radiation. We witnessed such an event in NGC 5548 and NGC 3783 where 90% of the soft X-rays are blocked by a dense gas stream close to the BLR. Our joint observations with XMM-Newton, NuSTAR and HST/COS showed UV broad absorption lines associated with the X-ray absorption and allowed us to characterise this unique obscuration event completely. We propose to investigate a similar event in another Seyfert 1 using the same instruments. Swift monitoring (98 ks) will be used to find the event, which will be characterized by joint ToO observations with XMM-Newton (150 ks), HST/COS (4 orbits) and NuSTAR (50 ks).

PROPOSAL: 084469     PI: Jian Li

Observing the X-ray emission of muon neutrino events

    We propose for one 30 ks XMM ToO observation and one joint 40ks NuSTAR ToO observation on one muon track neutrino event associated with a FAVA transient. The proposed joint XMM & NuSTAR observations will identify its X-ray counterpart and characterize its X-ray emission. Together with multi-wavelength follow-ups, this proposal aims to constrain and distinguish the neutrino & multiwavelength emission scenarios for new high energy neutrino sources.

PROPOSAL: 084526     PI: Luigi Piro

Observing the gravitational wave sky with XMM-Newton

    Our discovery of the first X-ray counterpart of a GW event, followed by a long-term multi-wavelength monitoring, provided the first compelling evidence of a relativistic outflow launched from the merger remnant and seen off-axis. During the next science run, planned to start in early 2019, the GW detectors will lead to routine detections of GW sources and their electromagnetic counterparts. We propose to exploit the unique capabilities of XMM-Newton to map out the diversity of these sources, and constrain the structure of their outflows. The next few detections of EM counterparts will shape our knowledge of GW sources for years to come, and the proposed observations will lead to seminal results in the nascent field of multi-messenger astronomy.

PROPOSAL: 084545     PI: James Court

XMM-Newton observation of highly structured flaring in the bursting pulsar

    The Bursting Pulsar (GRO J1744-28) is a neutron star low-mass X-Ray binary. In addition to showing Type II X-Ray bursts during outburst, at lower luminosities the source displays highly structured X-ray flaring remarkably similar to variability seen in the enigmatic black hole binary GRS 1915+105. The mechanism behind this variability is poorly understood, but believed to be related to the viscous instabilities which also give rise to Type II bursts. As such, the Bursting Pulsar is an excellent case study to study the relationship between these two behaviours. We propose two joint XMM-Newton and NuStar ToO observations totalling 80 ksec, separated by 15 days to capture both GRS 1915-like behaviour and the bursting behaviour which immediately precedes it.

PROPOSAL: 086127     PI: Allesandro Papitto

Hunting for transitional millisecond pulsars with XMM-Newton and NuStar

   The XMM discovery of a ms pulsar swinging between an accretion-powered (Xray) and a rotation-powered (radio) pulsar state demonstrated that transitions between the two states can be observed over timescales of a few weeks. We propose an XMM/NuStar ToO observation of 60 ks aimed at studying the accretion state of transitional ms pulsars, detecting X-ray accretion powered pulsations, and characterizing its variability over three decades in energy. Candidates are restricted to black widows and redbacks, systems in an evolutionary phase that allows state transitions. Enlarging the number of systems in this transitional phase is crucial to test binary evolution theories, and to study the disk-field interaction over a large range of mass accretion rates.

PROPOSAL: 086276     PI: Hannah Earnshaw

NGC 925 ULX-3: a key to understanding extreme ULX variability

   We propose a broadband observation of the transient ULX, NGC 925 ULX-3, for 40ks with XMM-Newton quasi-simultaneous with a 100ks observation with NuSTAR. This source exhibits extreme levels of long-term variability of over a factor of 36, a feature also seen in several ULXs identified as neutron stars to date and linked to different possible mechanisms that are not well understood. We plan to trigger this observation when the source exceeds a flux of 6x10^-13 ergs/s/cm2, determined from an existing Swift monitoring campaign, allowing us to constrain its high-flux broadband spectrum. Coupled with this monitoring and a complementary proposal at a low flux, we will be able to build an understanding of the geometry and physical processes in play in this extreme accreting system.

PROPOSAL: 086342     PI: Raffaella Margutti

Detailed mapping of extreme mass loss with coordinated XMM-NuSTAR observations

   Contrary to expectations from current stellar evolutionary models, recent observations uncovered the ejection of shells of material by massive stars in the years before the supernova (SN) explosion. The physical mechanism behind the impulsive mass ejection synchronized with the stellar core-collapse is unclear. Here we propose a coordinated XMM-NuSTAR effort to map the evolution of the broad-band X-ray spectrum of 1 nearby (d< 50 Mpc) strongly interacting SN and enable progress. Our program has the immediate goal to characterize the medium around strongly interacting SN, which originate from stellar progenitors with the most extreme mass loss before explosion.

PROPOSAL: 086403     PI: Jennifer Sokoloski

Preparing for the next Nova of the Century

   Changes in the amount of material reaching the accretion disk boundary layer can drastically modify its structure. We are witnessing such changes in a symbiotic recurrent nova, T CrB, for the first time. Two observations with NuSTAR and XMM have revealed that the accretion rate has increased by a factor of about 10 and that the boundary layer has become optically thick. The next step toward our understanding of the current state is to observe T CrB when it returns to its quiescence level, which may happen during AO19. This ToO observation will allow us to test the idea that nova eruptions in T CrB follow after multi-year periods of enhanced accretion onto the white dwarf due to an accretion disk instability. It will also help us study disk instablity events in large accretion disks.

PROPOSAL: 086460     PI: Nanda Rea

Prompt XMM-Newton and NuSTAR study of magnetar outbursts

   The magnetars are neutron stars mainly powered by their huge magnetic fields. They go through long stretches of quiescence, interrupted by periods of activity, via short X-ray bursts, large giant flares and yearly-timescale X-ray outbursts. Lately several normal pulsars showed magnetar-like emission, prompting important questions on the exact physics and reach of the magnetar phenomenon. We propose a Joint XMM (85ks) and NuSTAR (170ks) ToO program aimed at gathering new insights on the physics involved in the magnetar outbursts. We will trigger this program on any object that will undergo a magnetar-like outburst during AO19 (especially non-canonical magnetars).

PROPOSAL: 088008     PI: Norbert Schartel

Radio-quiet AGN in outburst

   XMM-Newton observations of Seyfert galaxies established outbursts of radio-quiet AGNs as a still poorly explored discovery space for AGN physics with an enormous potential to learn: NGC 1566, HE1136-2304 and NGC 2617 are rare and extreme changing-look AGNs tracing rapid changes in accretion and/or corona physics including the launching of new outflows. GSN 069 very likely traces an AGN disturbed by a tidal disruption event. We propose two 80ks simultaneous XMM-Newton(EPIC and RGS)/NuSTAR observations accompanied by optical spectroscopy of the next suited AGN outburst(s). They will allow tracing the accretion physics, the launching of winds and outflows, the spectral complexity connected with dramatic emission-line changes and possibly the elusive tidal disruption events.

PROPOSAL: 088054     PI: Pierre-Olivier Petrucci

Tracking a Black Hole Low-Mass X-ray Binary Wind during Spectral Transition

   We propose a Large Program of 10-day XMM-NuSTAR consecutive monitoring (30 ks/day XMM, 20 ks/day NuSTAR), triggered by Swift, of a high-inclination Black Hole LMXB during the Hard-to-Soft transition. This will allow us to simultaneously and accurately 1) determine the X-ray broadband spectral shape and 2) detect the wind absorption lines. These are key ingredients to put firm constraints on (a) the evolution of the wind physical properties with the accretion state and (b) its possible association with the jet (thanks to proprietary simultaneous IR-Radio observations).

PROPOSAL: 088218     PI: Nanda Rea

Prompt XMM-Newton and NuSTAR study of magnetar outbursts

   Magnetars are neutron stars mainly powered by their huge magnetic fields. They go through long stretches of quiescence, interrupted by periods of activity, via short X-ray bursts, large giant flares and yearly-timescale X-ray outbursts. Lately several normal pulsars showed magnetar-like emission, and an FRB-like event was observed during a short X-ray burst, prompting important questions on the exact physics and reach of the magnetar phenomenon. We propose a Joint XMM (85ks) and NuSTAR (170ks) ToO program aimed at gathering new insights on the physics involved in the magnetar outbursts. We will trigger this program on any object that will undergo a magnetar-like outburst during AO20 (especially non-canonical magnetars or FRB-like emitters).

PROPOSAL: 088470     PI: Alessandro Papitto

X-ray pulse waveform from an accreting ms pulsar and the equation of state

   Modelling of the waveform of the X-ray pulsations of accreting ms pulsars (AMSP) is one of the most promising ways to constrain the equation of state of neutron stars. However, it requires an extremely high number of counts to break the degeneracy between the many parameters that shape the X-ray pulse profiles. The polarimetric information granted by the forthcoming IXPE mission will measure the geometry of the hot spots independently, easing the requirement. We propose a 120 ks XMM-NuSTAR ToO observation of the next outburst of an AMSP to measure the pulsar ephemeris and fold simultaneous IXPE data, model the broadband spectrum, and derive a high statistics energy-resolved pulse profile. The proposed observation will measure the mass and the radius with an accuracy of a few per cent.

PROPOSAL: 090022     PI: Norbert Schartel

Catching AGN in Deep Minimum States to Unveil Their Core Environment

   Beside a serendipitous approach (e.g. ATel) we search systematically using two resources: Swift Fill-in and serendipitous observations of AGNs (Grupe et al. 2010, ApJS, 187, 64): based on previous years experience and taking into account XMM-Newton visibility we expect to detect 1.5 low-state AGN visible per year. XMM-Newton slews: All slew sources are public in the XMM-Newton web-page within 1 - 2 weeks. We check all sources found in XMM-Newton slews and search for AGNs which are at least 10 times fainter than they were during ROSAT measurements. We expect one candidate every two years. Depending on the count rate detected by Swift or XMM-Newton slew we will decide about the (short) XMM-Newton follow-up observation duration. If the short XMM-Newton observation shows spectral complexity, then we plan to trigger a further, deeper observation with an exposure time of 80 ks. Contemporaneously we want to trigger a simultaneous 80ks NuSTAR observation and we want to trigger a 2 orbit long HST Cosmic Origins Spectrograph (COS) observation with E130M and G160M. The reaction time for the triggers can be some weeks depending on the visibility of the target.

PROPOSAL: 090023     PI: Norbert Schartel

The extreme X-ray weakness of PG 0043+039

   PG 0043+039 is the most X-ray weak quasar known. XMM-Newton observation in 2013 set a clear detection meaning a flux at least 3.8 times higher than in 2005. The quasar remained extremely X-ray weak, with a peculiar spectral energy distribution (SED) and a unique UV spectrum. This unique source deserves further, deeper, observations, to establish its X-ray spectrum, its optical to hard-X-ray SED and its variability. We aim at probing our hypothesis that PG 0043+039 is a genuine X-ray weak quasar and to test alternative scenarios (ADAFs, missing corona, cyclotron-emission). We propose to observe PG0043+039 quasi-simultaneously (2 days) with XMM-Newton (20+100ks), NuSTAR (60ks), HST (2 orbits), 10m Hobby-Eberly and 10m SALT Telescope in South Africa.

PROPOSAL: 090067     PI: Adam Ingram

The first X-ray polarimetry and spectroscopy observations of GRS 1915+105

   We propose three 30ks XMM-Newton/EPIC-pn exposures contemporaneous with three 20ks NuSTAR exposures of the black hole (BH) X-ray binary (XRB) GRS 1915+105, to be taken during an IXPE (the Imaging X-ray Polarimetry Explorer) observation of the source. A 250ks IXPE exposure of GRS 1915+105 is scheduled for April 2022, and we have an accepted AO20 XMM+NuSTAR (ID 088100) proposal. The current proposal is only to cover the eventuality of a delay to the IXPE programme pushing the GRS 1915+105 observation into the AO21 cycle. The high spectral resolution and broad band pass provided by XMM+NuSTAR coverage will be vital for breaking degeneracies in our polarization models. We will use MAXI and Swift/XRT monitoring to determine whether to setup the EPIC-pn in timing or burst mode.

PROPOSAL: 090245     PI: Missagh Mehdipour

Tackling unresolved questions on transient obscuring outflows in AGN

   Transient obscuring outflows are remarkably different from the commonly seen warm-absorber outflows in AGN. They exhibit large columns of high-velocity gas near the accretion disk/BLR. They shield much of the X-ray radiation, which can have important implications for the surrounding gas and the launching of AGN outflows. Joint ToO observations with XMM-Newton, NuSTAR, and HST, triggered using Swift monitoring, have been vital for ground-breaking results on transient obscuration events to come into light in recent years. We propose to study a new event to address the outstanding questions on the link between obscuration and AGN outflows and broaden our understanding of these events. We thus request joint ToO observations with XMM-Newton (60 ks), NuSTAR (30 ks), and HST/COS (2 orbits).

PROPOSAL: 090251     PI: Daniel Stern

Extreme AGN Fading from eROSITA

   Spectrum-Roentgen Gamma (SRG) is, for the next two years, providing the unique ability to identify highly variable X-ray sources across the full sky at depths significantly fainter than Swift/BAT or MAXI. We propose here for six XMM-Newton + NuSTAR coordinated target-of opportunity (ToO) observations of AGN that have shown significant (>10x) fading between eROSITA passes. Broad-band data from XMM-Newton + NuSTAR will robustly determine whether the fading is due to variable extinction, which provides a probe of the clumpy torus structure, or a true decrease in the intrinsic X-ray emission due to fading or collapse of the central engine.

PROPOSAL: 090284     PI: Javier Garcia

Truncated Accretion Disks in Black Hole Binaries

   We request 80 ks of XMM-Newton and 40 ks of NuSTAR time to trigger Target of Opportunity (ToO) observations of a new or known black hole transient during the end of its outburst, after the source has entered the low-hard state. We aim to obtain high signal-to-noise data during this fainter phase in order to measure the level of disk truncation using X-ray reflection spectroscopy. We will also provide measurements for the disk inclination, ionization and iron abundance. These observations will provide crucial constraints to support a large-scale data analysis program for these sources.

PROPOSAL: 090332     PI: Rafaella Margutti

XMM-NuSTAR Observations of Rapid, Luminous and Blue Stellar Explosions

   We propose an in-depth X-ray study of fast blue optical transients (FBOTs), a new class of transients with luminosities and time scales that challenge traditional SN models. Alternative scenarios include a failed SN of a stripped star, He-shell detonation on a white dwarf, and a SN shock breaking through a dense medium. FBOTs have mostly been studied in the optical/UV regime, which is of thermal origin and it is not sensitive to the nature of the underlying energy source. Here we propose to capitalize on our recent discovery of X-rays from the FBOT AT2018cow, AT2020xnd and CSS161010 with unprecedented properties, and start the first broad-band X-ray monitoring campaign of FBOTs. The primary goal is to test for the presence of engines driving the explosions and constrain their nature.

PROPOSAL: 090357     PI: Sergio Campana

XMM-Newton follow-up of electromagnetic counterparts to GW triggers during LVKC-O4

   The joint detection of GWs and light from the NS merger GW170817 ushered us into a new era. Deep X-ray observations of GW170817 over 4 yr constrained the structural properties of a relativistic outflow launched by an NS merger and possibly detected the first evidence for the emerging kilonova afterglow. After this landmark discovery, the frontier is now to map the properties of a population of GW sources. We propose to exploit the unique capabilities of XMM-Newton to constrain the broad-band properties of 2 GW sources detected during LVKC-O4. Our immediate goals are: (i) map the diversity of emission from NS mergers, (ii) test the nature of the remnant object, (iii) determine if all mergers launch a jet, (iv) enable the first X-ray detections of NS-BH mergers or unclassified GW sources.

PROPOSAL: 092118     PI: Raffaella Margutti

Extreme Mass Loss from Massive Stars constrained by XMM-NuSTAR observations

   Contrary to expectations from current stellar evolutionary models, recent observations uncovered the ejection of shells of material by massive stars in the years before the supernova (SN) explosion. The physical mechanism behind the impulsive mass ejection synchronized with the stellar core-collapse is unclear. Here we propose a coordinated XMM-NuSTAR effort to map the evolution of the broad-band X-ray spectrum of 1 nearby (d< 50 Mpc) or X-ray bright (Fx>3 x 10^-13 erg/s/cm2) strongly interacting SN and enable progress. Our program has the immediate goal to characterize the medium around strongly interacting SNe, which originate from stellar progenitors with the most extreme mass loss before explosion.

PROPOSAL: 092270     PI: Elias Kammoun

A deep look at the heart of the most luminous QSO in the last 9 Gyr

   Rapidly accreting QSOs are unique laboratories to understand supermassive black hole growth, and AGN feedback. Recent observations show that surprisingly, these sources do not differ much from slowly accreting QSOs. Some of these sources show a unexpected variability, notably in X-rays. Results from eROSITA and our recent monitoring of SMSS J1144 (z=0.83, log M/Msun = 9.4, Lbol/LEdd = 1.4) show that this QSO is highly variable on timescales ranging from weeks to a few months. We propose a Swift monitoring of the source that will allow us to trigger two XMM-Newton + NuSTAR ToO to catch the source in the low- and high-flux states. This will allow us to better measure the properties of the source, identify signs of reflection and feedback, and understand the origin of the unusual variability.

PROPOSAL: 092317     PI: Alessandro Papitto

X-ray pulse waveform from an accreting ms pulsar and the equation of state

   Modelling of the waveform of the X-ray pulsations of accreting ms pulsars (AMSP) is one of the most promising ways to constrain the equation of state of neutron stars. However, it requires an extremely high number of counts to break the degeneracy between the many parameters that shape the X-ray pulse profiles. The polarimetric information granted by the forthcoming IXPE mission will measure the geometry of the hot spots independently, easing the requirement. We propose a 120 ks XMM-NuSTAR ToO observation of the next outburst of an AMSP to measure the pulsar ephemeris and fold simultaneous IXPE data, model the broadband spectrum, and derive a high statistics energy-resolved pulse profile. The proposed observation will measure the mass and the radius with an accuracy of a few per cent.

PROPOSAL: 092331     PI: Missagh Mehdipour

Catching changing-look NGC 3516 at a bright flux to establish the role of winds

   NGC 3516 is a remarkable Seyfert-1 galaxy, which over the last decade has undergone major spectral transformations, classifying it as a changing-look AGN. Understanding the nature and origin of spectral variability in such changing-look AGN would provide key insights into the accretion-outflow mechanism. Swift monitoring of NGC 3516 shows dramatic flux rises and declines during its changing-look state. We propose ToO observations with XMM, NuSTAR, and HST, triggered using Swift monitoring, to catch NGC 3516 at a high flux level to be able to carry out the first high resolution X-ray and UV spectroscopy of its winds during the changing-look state. The proposed observations would enable us to ascertain the role of winds in the transformation of NGC 3516.

PROPOSAL: 092346     PI: Arianna Miraval Zanon

Hunting for transitional millisecond pulsars with XMM-Newton, NuSTAR and VLA

   The XMM discovery of a ms pulsar swinging between an accretion-powered (X-ray) and a rotation-powered (radio) pulsar state demonstrated that transitions between the two states can be observed over timescales of a few weeks. We propose an XMM, NuSTAR and VLA ToO observation aimed at studying the accretion state of transitional ms pulsars, detecting X-ray accretion powered pulsations, and characterizing the X-ray variability over three decades in energy. Candidates are restricted to black widows and redbacks, systems that are in an evolutionary phase that makes them good candidates to observe a transition. Enlarging the number of transitional ms pulsars is crucial to test binary evolution theories, and to study the disk-field interaction over a large range of mass accretion rates.

PROPOSAL: 092410     PI: Erin Kara

Mapping the evolving gas flows in the AGN Mrk 817 with XMM-Newton and HST

   Knowledge of the structure and kinematics of gas around supermassive black holes is vital to understanding accretion and thus AGN feedback. To this end, we, the AGN STORM 2 team, have conducted the largest multi-mission and multi-wavelength campaign of an AGN to date. This includes a Large 198-orbit Hubble Space Telescope Program, an XMM-Newton Program (AO20), a Swift Key Project, and monitoring with NICER and optical ground based facilities. The prime AGN STORM 2 campaign (from Dec 2020-Feb 2022) revealed a wealth of information on the structure of the broad-emission-line regions (BLR), accretion disk, and new, unexpected obscuring outflows visible in X-ray and far-UV spectra. The success of this campaign has led to the extension of the Swift, HST and groundbased campaigns, all of which are ongoing. Here, we request to also extend the XMM-Newton+NuSTAR campaign, with one 120 ks orbit during AO22, and potentially a 120 ks ToO observation (accompanied with pre-approved HST COS ToO) if the AGN enters an unobscured state. AGN STORM 2 is only the 2nd 200-orbit HST reverberation campaign in its 30 years mission lifetime, and will be challenging to replicate once HST enters reduced gyro mode in a few years.

PROPOSAL: 094004     PI: Norbert Schartel

Radio-quiet AGN in outburst

   XMM-Newton observations of Seyfert galaxies established outbursts of radio-quiet AGNs as a still poorly explored discovery space for AGN physics with an enormous potential to learn: NGC 1566, HE1136-2304 and NGC 2617 are rare and extreme changing-look AGNs tracing rapid changes in accretion and/or corona physics including the launching of new outflows. Sources like GSN 069 very likely traces an AGN disturbed by a tidal disruption event. We propose two 80ks simultaneous XMM-Newton(EPIC&RGS)/NuSTAR observations accompanied by optical spectroscopy of the next suited AGN outburst(s). They will allow tracing the accretion physics, the launching of winds and outflows, the spectral complexity connected with dramatic emission-line changes and possibly the elusive tidal disruption events.

PROPOSAL: 094084     PI: Rafaella Margutti

XMM-NuSTAR observations of the most Extreme Mass Loss events from Massive Stars

   Contrary to expectations from current stellar evolutionary models, recent observations uncovered the ejection of shells of material by massive stars in the years before the supernova (SN) explosion. The physical mechanism behind the impulsive mass ejection synchronized with the stellar core-collapse is unclear. Building on our recent success with SN2023ixf, here we propose a coordinated XMM-NuSTAR effort to map the evolution of the broad-band X-ray spectrum of 1 nearby (d< 50 Mpc) or X-ray bright (Fx>3 x 10^-13 erg/s/cm2) strongly interacting SN and enable progress. Our program has the immediate goal to characterize the medium around strongly interacting SNe, which originate from stellar progenitors with the most extreme mass loss before explosion.

PROPOSAL: 094134     PI: Erin Kara

Mapping the evolving gas flows in the AGN Mrk 817 with XMM-Newton and HST

   The AGN STORM 2 Team has conducted the largest, cadenced multi-wavelength campaign of an AGN to date. This includes a Large HST Program, a Large XMM Program (AO20), a Swift Key Project, NICER and ground based monitoring. The campaign (which started in Dec 2020) has revealed a wealth of information on the structure and dynamics the AGN, including an unexpected obscuring outflows visible in X-ray and far-UV spectra. The success of this campaign has led to the continuation of the campaign. Now, we appear to be entering a new phase, where the obscuration from the outflow is diminishing. Here, we request to also extend the XMM-Newton+NuSTAR campaign, with one 120 ks ToO observation to catch the AGN in a new, truly unobscured state. This observation would take place with a pre-approved HST observation.

PROPOSAL: 094209     PI: Michael Nowak

Searching for a Wind in the Bright State of 4U1957+11

   How does a black hole transit from a jet-producing system in a spectrally hard state to a soft state? Under what conditions does this state produce winds, and when can we observe these winds? Are wind launching mechanisms dominated by radiative, thermal, or magnetic driving? Seeking answers to these questions, we propose a triggered XMM/NuSTAR observation of the persistently bright LMXB 4U1957+11, which is hypothesized to harbor a near-maximally spinning black viewed close to edge-on. It has been observed with high resolution X-ray spectroscopic instruments in low flux/minimal hard tail states. We will use MAXI monitoring to target a bright soft state with a significant non-disk contribution, potentially indicative of an outflowing corona/wind, and search for spectroscopic signatures of a wind.

PROPOSAL: 094218     PI: Murray Brightman

Hunting for the wind features in NGC 4045 ULX with XMM-Newton and NuSTAR

   Observations of a newly discovered hyperluminous X-ray source in the galaxy NGC 4045 at a distance of 32 Mpc have revealed an absorption line at E=8.56 keV detected with a significance of > 3.3 sigma. The origin of the absorption line appears likely to be by highly ionized iron with a blue shift of 0.2c, indicating an ultrafast outflow. However, the large equivalent width of the line, EW=-0.22 keV, requires extreme wind parameters. An alternative explanation is that the line is due to a cyclotron resonance scattering feature, produced by the interaction of X-ray photons with the powerful magnetic field of a neutron star. The goal of this proposal is to detect the additional wind features, such as a P-Cygni line profile, with a target of opportunity observation with XMM-Newton and NuSTAR.

PROPOSAL: 094376     PI: Samuel Barnier

ToO : Comparing the early rise and decay phases of X-ray binaries outbursts

   Recent studies suggest different behaviours between the rising and decaying hard states of black hole X-ray binaries outbursts. However the nature of this change eludes us, mainly due to a lack of data. We propose a ToO monitoring of a future outburst of a known BH XrB composed of 10 XMM/NuSTAR simultaneous observations, 10 ks/obs for XMM + 20 ks/obs NuSTAR (total of 100ks XMM and 200ks NuSTAR) to catch the early rising (5 observations) and decaying phases (5 observations) in the same hard X-ray flux range. The trigger conditions will be based on the "X-ray Binary New Early Warning System" in optical, anticipating X-ray outbursts by a few days/weeks. Quasi-simultaneous ATCA and MeerKAT observations are planned.

PROPOSAL: 094484     PI: Maxime Parra

Tracking the Wind of a Black Hole Low-Mass X-ray Binary in Spectral Transition

   Winds in low mass X-ray binaries (LMXB) might carry away a significant amount of matter and are thus able to regulate the evolution of these systems. To make a decisive breakthrough in our understanding of these winds, we propose a Large Program of 10-day XMM-NuSTAR consecutive monitoring (30 ks/day XMM, 20 ks/day NuSTAR), triggered by Optical and X-ray alerts, of a high-inclination Black Hole LMXB during the Hard-to-Soft transition. This will allow us to simultaneously and accurately determine the X-ray broadband spectral shape and detect the wind absorption lines. These are key ingredients to put firm constraints, both on the evolution of the wind physical properties with the accretion state and its possible association with the jet (thanks to proprietary simultaneous IR/radio observations).

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Joint NuSTAR / INTEGRAL

PROPOSAL: 1540022     PI: Sergey Tsygankov

Measuring the High Energy Emission of Millisecond X-Ray Pulsars in Outburst

    We propose to perform two 170 ks INTEGRAL ToO observations of a transient millisecond X-ray pulsar in outburst. To constrain the broadband spectrum we also request one 30 ks simultaneous NuSTAR and two 50 ks XMM-Newton observations. The target can be either one of the fifteen known transient accreting millisecond X-ray pulsars (AMXPs) in the case of a new outburst or a "newly" observed object of this class. These observations allow us to study the broad band spectrum in detail from hard to soft X-ray energies as well as the timing properties or eclipsing features of the source during its outburst. The high-statistics spectral information make it possible to disentangle the contributions of soft black body, reflection (if any) and hard Comptonized spectral components. Moreover, we will detect type-I X-ray bursts, and/or for the first time also burst oscillations at high-energy (if present). In particular we will be able to study the energy spectrum in a broad energy range (0.1-300 keV) and with unprecedented high-sensitivity above 20 keV. The INTEGRAL and XMM-Newton observations will also allow a timing analysis to study the pulse profile, time lags and pulsed spectrum, and so will provide important constraints on emission mechanisms. Similar proposals were approved for the INTEGRAL AO2-AO14 cycles, and during these years our ToO has been triggered eight times, i.e., in 2003 we observed XTE J1807-294, in 2004, 2005, 2009, 2011, and 2013 we triggered on newly discovered sources IGR J00291+5934, HETE 1900.1-2455, IGR J17511- 3057, IGR J17498-2921, and IGR J18245-2452, respectively. In 2015, we triggered on IGR J00291+5934 and SAX J1748.5-2021 (data have been made public, because it was not clear whether the source in outburst was the AMXP).

PROPOSAL: 1640015     PI: Ingo Kreykenbohm

New cyclotron lines in transient pulsars

    We propose a Target of Opportunity observation of a transient accreting X-ray pulsar during a bright outburst for which no cyclotron lines are known so far. We aim to detect cyclotron lines in transient pulsars to extend the list of sources with known cyclotron lines. We propose to perform a 400 ksec long single observation during the peak of the outburst to obtain a spectrum with a very high signal to noise ratio to be able to search also for rather shallow cyclotron lines or CRSFs at energies above 50 keV in the steeply falling part of the continuum. Furthermore we propose to coordinate the INTEGRAL observation with a 20 ksec long NuStar observation to cover also the energy range between 10 keV and 25 keV.We propose a Target of Opportunity observation of a transient accreting X-ray pulsar during a bright outburst for which no cyclotron lines are known so far. We aim to detect cyclotron lines in transient pulsars to extend the list of sources with known cyclotron lines. We propose to perform a 400 ksec long single observation during the peak of the outburst to obtain a spectrum with a very high signal to noise ratio to be able to search also for rather shallow cyclotron lines or CRSFs at energies above 50 keV in the steeply falling part of the continuum. Furthermore we propose to coordinate the INTEGRAL observation with a 20 ksec long NuStar observation to cover also the energy range between 10 keV and 25 keV.

PROPOSAL: 1640021     PI: Julia Alfonso-Garzon

Studying the connection between the circumstellar disk and the accretion rate for the Be/XRB system H 1145-619

    H 1145-619 is a Be/X-ray binary pulsar which displayed frequent, bright outbursts in the 70s to 90s. However, since the advent of modern X-ray astronomy it has remained in a nearly quiescent state. We have compiled the optical and X-ray light evolution of this system, spanning ~40 years, and have found signs of an ongoing replenishment of the Be circumstellar disk. This replenishment suggests that a bright X-ray outburst may happen during the next periastron passages. Therefore, in this proposal we solicit TOO observations of the system in case an X-ray outburst is detected during the 6 April or 9 October 2019 periastron passages. We request to perform one INTEGRAL 170 ks observation during the rising phase of the outburst and one NuSTAR 40 ks observation close to the peak of the outburst, complemented by Swift/XRT monitoring in soft X-rays (seven 1 ks observations every two days). With these observations we expect to model the broad-band spectrum of the source with newly developed physical models and to study the pulse profile variations in different energy bands. We will compare our results with those from previous X-ray outbursts from the 90s, obtained with poorer spectral resolution, and search for cyclotron lines, not detected so far in this system. We will also study the absorption column and spectral variations over the outburst, and determine the orbit elements of the system. The Optical Monitoring Camera (OMC) onboard INTEGRAL will provide simultaneous optical photometry of the source. To complement these observations, we plan to perform simultaneous H-alpha spectroscopy with SALT, to characterize the evolution of the Be disk.

PROPOSAL: 1740029     PI: Riccardo La Placa

Measuring the High Energy Emission of Millisecond X-Ray Pulsars in Outburst

    We propose to perform two 170 ks INTEGRAL ToO observations of a transient millisecond X-ray pulsar in outburst.To constrain the broadband spectrum we also request one 30 ks simultaneous NuSTAR and two 50 ks XMM-Newton observations. The target can be either one of the eighteen known transient accreting millisecond Xray pulsars (AMXPs) in the case of a new outburst or a "newly" observed object of this class. These observations allow us to study the broad band spectrum in detail from hard to soft X-ray energies as well as the timing properties or eclipsing features of the source during its outburst. The high-statistics spectral information make it possible to disentangle the contributions of soft black body, reflection (if any) and hard Comptonized spectral components. Moreover, we will likely detect type-I X-ray bursts, and/or for the first time also burst oscillations at high-energy (if present). In particular we will be able to study the energy spectrum in a broad energy range (0.1-300 keV) and with unprecedented high-sensitivity above 20 keV. The INTEGRAL and XMM-Newton observations will also allow a timing analysis to study the pulse profile, time lags and pulsed spectrum, and so will provide important constraints on emission mechanisms. Similar proposals were approved for the INTEGRAL AO2-AO16 cycles, and during these years our ToO has been triggered eight times, i.e., in 2003 we observed XTE J1807-294, in 2004, 2005, 2009, 2011, and 2013 we triggered on newly discovered sources IGR J00291+5934, HETE J1900.1-2455, IGR J17511- 3057, IGR J17498-2921, and IGR J18245-2452, respectively. In 2015, we triggered on IGR J00291+5934 and SAX J1748.9-2021, and in 2018 we triggered again on a newly discovered source IGR J17591-2342.

PROPOSAL: 1740033     PI: Julia Alfonso-Garzon

Studying the connection between the circumstellar disk and the accretion rate for the Be/XRB system H 1145-619

    H 1145-619 is a Be/X-ray binary pulsar which displayed frequent, bright outbursts in the 70s to 90s. However, since the advent of modern X-ray astronomy it has remained in a nearly quiescent state. We have compiled the optical and X-ray light evolution of this system, spanning ~40 years, and have found signs of an ongoing replenishment of the Be circumstellar disk. This replenishment suggests that a bright X-ray outburst may happen during the next periastron passages. Therefore, in this proposal we solicit TOO observations of the system in case an X-ray outburst is detected during the 6 April or 9 October 2019 periastron passages. We request to perform one INTEGRAL 170 ks observation during the rising phase of the outburst and one NuSTAR 40 ks observation close to the peak of the outburst, complemented by Swift/XRT monitoring in soft X-rays (seven 1 ks observations every two days). With these observations we expect to model the broad-band spectrum of the source with newly developed physical models and to study the pulse profile variations in different energy bands. We will compare our results with those from previous X-ray outbursts from the 90s, obtained with poorer spectral resolution, and search for cyclotron lines, not detected so far in this system. We will also study the absorption column and spectral variations over the outburst, and determine the orbit elements of the system. The Optical Monitoring Camera (OMC) onboard INTEGRAL will provide simultaneous optical photometry of the source. To complement these observations, we plan to perform simultaneous

PROPOSAL: 1840002     PI: Thomas Siegert

R-Process Elements from Neutron Star Mergers

    The first simultaneous detection of a long-lasting gravitational wave signal (GW170817), together with prompt electromagnetic emission only 1.7 s after (GRB170817A), proved the connection between binary neutron star mergers (NSMs) and short gamma-ray bursts (sGRBs). While the prompt emission is now believed to be fully explained by synchrotron radiation, the delayed emission days to weeks after the merger, is thought to originate from radioactive decays of freshly-produced r-process elements during the NSM. While these "kilonova" signatures have been measured only indirectly at UVOIR wavelengths, the direct proof of radioactivity being the true heating source is still missing. Similar to supernovae, merging NSs produce large amounts of nucleosynthesis products that are ejected into the interstellar medium. Because this environment is neutron-rich, fast neutron capture reactions dominate over beta-decays (r-process), and is expected to form characteristic abundance patterns with three distinct peaks at atomic mass numbers, A, around 80, 130, and 200. The decay of these isotopes happens on a time scale of days to weeks, and the emitted gamma-rays, in the range between ~3 and ~3000 keV, have a high chance to escape the NSM. Only INTEGRAL/SPI and NuSTAR are capable to observe this radiation and to distinguish between individual r-process elements by their characteristic gamma-ray lines. We propose a 1.5 Ms ToO observation with INTEGRAL of a nearby (< 1 Mpc) NSM, together with contemporaneous NuSTAR observations (100 ks), to directly measure r-process element nucleosynthesis.

PROPOSAL: 1840005     PI: Zhaosheng Li

Measuring the High Energy Emission of Millisecond X-Ray Pulsars in Outburst

    We propose to perform two 170 ks INTEGRAL target of opportunity (ToO) observations of a transient millisecond X-ray pulsar in outburst. To constrain the broadband spectrum we also request one 30 ks simultaneous NuSTAR and two 50 ks XMM-Newton observations. The target can be either one of the nineteen known transient accreting millisecond X-ray pulsars (AMXPs) under going a new outburst, or a "newly" discovered object of this class. These observations will allow us to study the broad band spectrum in detail, from hard to soft X-ray energies, as well as the timing properties or eclipsing features of the source during its outburst. The high signal-to-noise spectral information will make it possible to disentangle the contributions of soft black body, reflection (if any), and hard Comptonized spectral components. Moreover, we may detect type-I X-ray bursts, and/or for the first time also burst oscillations at high-energy (if present). In particular we will be able to study the energy spectrum in a broad energy range (0.1-300 keV) and with unprecedented high sensitivity above 20 keV. The INTEGRAL, NuSTAR, NICER, HXMT and XMM-Newton observations will also allow a timing analysis to study the pulse profile, time lags and pulsed spectrum, and will thus provide important constraints on emission mechanisms. Similar proposals were approved for the INTEGRAL AO2-AO17 cycles, and during these years our ToO has been triggered ten times: in 2003 we observed XTE J1807-294, in 2004, 2005, 2009, 2011, and 2013 we triggered on newly discovered sources IGR J00291+5934, HETE J1900.1-2455, IGR J17511-3057, IGR J17498-2921, and IGR J18245-2452, respectively. In 2015, we triggered on IGR J00291+5934 and SAX J1748.9-2021, and in 2018 we triggered again on a newly discovered source IGR J17591-2342 and Swift J1756.9-2508.

PROPOSAL: 1840021     PI: Ingo Kreykenbohm

Searching For New Cyclotron Lines in Transient Pulsars

    We propose a Target of Opportunity observation of a transient accreting X-ray pulsar during a bright outburst to study the flux dependency of a newly detected cyclotron line. Using our accepted NuSTAR TOO proposal, we will observe an accreting X-ray pulsar in outburst. If we detect an unknown cyclotron line in that NuSTAR observation, we propose to perform a 400 ksec long INTEGRAL observation during the peak of the outburst coordinated with another 40 ksec NuSTAR observation to obtain a spectrum with a very high S/N. The INTEGRAL and NuSTAR observations together will allow us to constrain the spectral continuum extremely well, permitting us to study any luminosity dependency of the cyclotron line and to find any harmonic line above 50 keV, in the steeply falling part of the continuum.

PROPOSAL: 1940020     PI: Ingo Kreykenbohm

Searching For New Cyclotron Lines in Transient Pulsars

   We propose a Target of Opportunity observation of a transient accreting X-ray pulsar during a bright outburst to study the flux dependency of a newly detected cyclotron line. Using our NuSTAR TOO proposal, we will observe an accreting X-ray pulsar in outburst. If we detect an unknown cyclotron line in that NuSTAR observation, we propose to perform a 400 ksec long INTEGRAL observation during the peak of the outburst (minimum: 200mCrab) coordinated with another 40 ksec NuSTAR observation to obtain a spectrum with a very high S/N. The INTEGRAL and NuSTAR observations together will allow us to constrain the spectral continuum extremely well, permitting us to study any luminosity dependency of the cyclotron line and to find any harmonic line above 50 keV, in the steeply falling part of the continuum.

PROPOSAL: 1940025     PI: Thomas Seigert

R-Process Elements from Neutron Star Mergers

   The first simultaneous detection of a long-lasting gravitational wave signal (GW170817), together with prompt electromagnetic emission only 1.7 s after (GRB170817A), proved the connection between binary neutron star mergers (NSMs) and short gamma-ray bursts (sGRBs). While the prompt emission is now believed to be fully explained by synchrotron radiation, the delayed emission days to weeks after the merger, is thought to originate from radioactive decays of freshly-produced r-process elements during the NSM. While these kilonova signatures have been measured only indirectly at UVOIR wavelengths, the direct proof of radioactivity being the true heating source is still missing. Similar to supernovae, merging NSs produce large amounts of nucleosynthesis products that are ejected into the interstellar medium. Because this environment is neutron-rich, fast neutron capture reactions dominate over beta-decays (r-process), and is expected to form characteristic abundance patterns with three distinct peaks at atomic mass numbers, A, around 80, 130, and 200. The decay of these isotopes happens on a time scale of days to weeks, and the emitted gamma-rays, in the range between ~3 and ~3000 keV, have a high chance to escape the NSM. Only INTEGRAL/SPI and NuSTAR are capable to observe this radiation and to distinguish between individual r-process elements by their characteristic gamma-ray lines. We propose a 1.5 Ms ToO observation with INTEGRAL of a nearby (< 1 Mpc) NSM, together with contemporaneous NuSTAR observations (100 ks), to directly measure r-process element nucleosynthesis.

PROPOSAL: 2040002     PI: Thomas Siegert

R-Process Elements From Neutron Star Mergers

   The first simultaneous detection of a long-lasting gravitational wave signal (GW170817), together with prompt electromagnetic emission only 1.7 s after (GRB170817A), proved the connection between binary neutron star mergers (NSMs) and short gamma-ray bursts (sGRBs). While the prompt emission is can be fully explained by synchrotron radiation, the delayed emission days to weeks after the merger, is thought to originate from radioactive decays of freshly-produced r-process elements during the NSM. While these kilonova signatures have been measured only indirectly at UVOIR wavelengths, the direct proof of radioactivity being the true heating source is still missing. Similar to supernovae, merging NSs produce large amounts of nucleosynthesis products that are ejected into the interstellar medium. Because this environment is neutron-rich, fast neutron capture reactions dominate over beta-decays (r-process), and is expected to form characteristic abundance patterns with three distinct peaks at atomic mass numbers, A, around 80, 130, and 200. The decay of these isotopes happens on a time scale of days to weeks, and the emitted gamma-rays, in the range between 3 and 3000 keV, have a high chance to escape the NSM. Only INTEGRAL/SPI, NuSTAR and the future COSI mission are capable to observe this radiation and to distinguish between individual r-process elements by their characteristic gamma-ray lines. We propose a 1.5 Ms ToO observation with INTEGRAL of a nearby (< 1 Mpc) NSM, together with contemporaneous NuSTAR observations (100 ks), to directly measure r-process element nucleosynthesis.

PROPOSAL: 2040026     PI: Maria Teresa Fiocchi

INTEGRAL-NuSTAR observation of an Ultra Compact X-ray Binaries in outburst

   The study of accretion onto neutron stars and black holes provides a unique window on the physics of strong gravity and dense matter. After the first detection of Gravitational Waves, a renewed interest has grown for the UltraCompact X-ray Binaries (UCXBs), which are likely sources of mHz gravitational waves. We request an INTEGRAL and NuSTAR ToO observation of an UCXB in outburst in a low/hard state (taken from a list of UCXB simultaneously visible by both satellite), for an exposure time of 300 ks and 25 ks for INTEGRAL and NuSTAR, respectively. Observations will be triggered following criteria based on hard X-ray brightening as detected by satellites in operation such as SWIFT, ASTROSAT, HXMT and INTEGRAL. Furthermore, in case of trigger we will also request Swift/XRT and Radio data from available facilities to follow the evolution in the soft X-ray and radio energy bands.The aim of this proposal is to identify the emission of each component as the accretion disk, the corona and the jet and to probe the physical parameters describing these emission components. These results together with comparison of previous data, will be a substantial building block for the improvement in our understanding of the UCXB properties and evolution. The previous spectral analysis and simulated spectra in a broad energy band has demonstrated that only combining Swift, INTEGRAL and NuSTAR instruments, we can discriminate the accretion disc, corona and jet emissions and monitor the physical parameters and timing behavior of a UCXB.

PROPOSAL: 2140005     PI: Maria Teresa Fiocchi

INTEGRAL-NuSTAR observation of an Ultra Compact X-ray Binaries in outburst

   The study of accretion onto neutron stars and black holes provides a unique window on the physics of strong Gravity and dense matter. After the first detection of Gravitational Waves, a renewed interest has grown for the Ultra-Compact X-ray Binaries (UCXBs), which are likely sources of mHz gravitational waves. We request an INTEGRAL and NuSTAR ToO observation of an UCXB in outburst in a low/hard state (taken from a list of UCXB simultaneously visible by both satellite), for an exposure time of 300 ks and 25 ks for INTEGRAL and NuSTAR, respectively. Observations will be triggered following criteria based on hard X-ray brightening as detected by satellites in operation such as SWIFT, ASTROSAT, HXMT and INTEGRAL. Furthermore, in case of trigger we will also request Swift/XRT and Radio data from available facilities to follow the evolution in the soft X-ray and radio energy bands. The aim of this proposal is to identify the emission of each component as the accretion disk, the corona and the jet and to probe the physical parameters describing these emission components. These results together with comparison of previous data, will be a substantial building block for the improvement in our understanding of the UCXB properties and evolution. The previous spectral analysis (e.g. for 1RXS J180408.9-342058 and 4U 1543-624) and simulated spectra in a broad energy band has demonstrated that only combining Swift, INTEGRAL and NuSTAR instruments, we can discriminate the accretion disc, corona and jet emissions and monitor the physical parameters and timing behavior of a UCXB. This observing AO21 proposal is an updated version of the approved and no-triggered proposal in previous AO20.

PROPOSAL: 2140012     PI: Ingo Kreykenbohm

Searching For New Cyclotron Lines in Transient Pulsars

   We propose a Target of Opportunity observation of a transient accreting X-ray pulsar during a bright outburst to look for new or unknown harmonic cyclotron lines. We propose to perform a 400 ksec long INTEGRAL observation during the peak of the outburst coordinated with a 40 ksec NuSTAR observation to obtain a spectrum from .5 keV to &100 keV with a very high S/N: while NuSTAR is the perfect choice below .50 keV, INTEGRAL will considerably extend the spectral coverage to higher energies. The INTEGRAL and NuSTAR observations together will therefore allow us to constrain the spectral continuum extremely well, which is crucial to find also a shallow harmonic line in the NuSTAR energy range or a line above 50 keV in the exponentially falling part of the continuum which would not be possible with NuSTAR or INTEGRAL alone.

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Joint NuSTAR / Swift

PROPOSAL: 1417159     PI: Marianne Vestergaard

ToO monitoring of the re-awakening AGN in MRK590: Catching the monster in the act!

    After a 10-year hiatus, Mrk590 appears to be rekindling its nuclear activity! Existing Swift data strongly suggest that the AGN is likely to turn on again in the near future. We wish to seize this rare and fortuitous opportunity to document, for the first time ever, the onset of AGN activity since this can lead to significant insight on the long-standing issue of how AGNs are fueled. Once triggered by another program (PI:Vestergaard) we ask to first observe Mrk590 ~daily with Swift for up to 60 days and then for 5 ~bi-weekly follow-up observations to establish if Mrk590 has entered a stable high or low flux state. We also ask for two 20ks NuSTAR observations to place tighter constraints on the potential emergence of a weak soft X-ray component and perhaps a high-energy reflection component.

PROPOSAL: 1619112     PI: Luigi Piro

CLOSING IN ON THE PROGENITORS OF FAST RADIO BURSTS

    Despite rapid observational progress, the nature of fast radio bursts and their progenitors remains unknown. We propose a multi-wavelength follow-up campaign in order to detect their counterparts and narrow down the range of possible models, closing in on their elusive progenitors. Rapid observations at X-ray and radio wavelengths are requested to search for a transient, short-lived afterglow counterpart. Late-time X-ray observations could instead unveil the smoking gun signature of a magnetar-powered nebula.

PROPOSAL: 1619139     PI: Liliana Sandoval

UNDERSTANDING THE ORIGIN OF FAST-EVOLVING LUMINOUS TRANSIENTS

    In 2018 the discovery of the transient AT2018cow was a big breakthrough in the astronomical community given its unprecedented characteristics. It was the first Fast-Evolving Luminous Transient (FELT) to be discovered in real time and extensively studied in all bands, which has led to several interpretations about its origin. Using Swift XRT and UVOT data, our team discovered the X-ray and UV emission of AT2018cow, showing it was a bright X-ray and UV transient with very rapid X-ray variability (of even hrs) and a peculiar X-ray spectrum. These characteristics were observed for the first time on a FELT. We propose to investigate the origin of FELTs by exploiting the rapid response and multi band coverage of Swift, the higher X-ray energy coverage of NuSTAR and the large sensitivity of VLA.

PROPOSAL: 1821098     PI: Kirill Sokolovsky

IN SEARCH OF SHOCKS IN NOVAE

    The theory predicts that shocks that power GeV $gamma$-rays in novae should emit most of their energy as X-rays. However the predicted bright X-ray emission was never observed. Whether this lack points to something fundamental about the shock physics, or is simply an issue of small-number statistics, is unclear. We request Swift monitoring (40ks split across 10 epochs) and NuSTAR snapshot (40ks single epoch) of a newly discovered bright Galactic nova to expand the sample of gamma-ray novae with exquisite X-ray light curves. The resulting data will be interpreted alongside multi-wavelength observations to better understand shocks in novae, particularly characterize the luminosity, duration, temperature, location, and absorbing column associated with these shocks.

PROPOSAL: 1821161     PI: Maria-Carla Nicotra

CLOSING IN ON THE PROGENITORS OF FAST RADIO BURSTS

    Despite rapid observational progress, the nature of fast radio bursts and their progenitors remains unknown. We propose a multi-wavelength follow-up campaign in order to detect their counterparts and narrow down the range of possible models, closing in on their elusive progenitors.

PROPOSAL: 1922051     PI: Liliana Rivera Sandoval

INVESTIGATING THE ORIGIN OF FAST-EVOLVING LUMINOUS TRANSIENTS

    In 2018 the discovery of the optical transient AT2018cow was a big breakthrough in the astronomical community given its unprecedented characteristics. It was the first Fast-Evolving Luminous Transient (FELT) to be discovered in real time and extensively studied at all wavelengths, which has led to several interpretations about its origin. Using Swift XRT and UVOT observations, it was determined that AT2018cow was a bright X-ray and UV transient with rapid X-ray variability, with timescales of even a few hours, and a peculiar X-ray spectrum. These characteristics were observed for the first time on a FELT. We propose to investigate the origin of FELTs by exploiting the rapid response and multi band coverage of Swift joined with the great capabilities of NuSTAR and the VLA.

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Joint NuSTAR / NICER

NICER PROPOSAL: 02122     PI: Melania Nynka

Long-term nicer monitoring of magnetar x-ray outbursts

    We propose a monitoring program using the soft X-ray instrument NICER to monitor the evolution of magnetar outbursts, two ToO triggers of 80ks each. With a coordinated NuSTAR observations (40ks), we will monitor the evolution of the pulse profile and spectrum over the decay phase of a magnetar outburst to provide crucial insight into the magnetic energy dissipation process in magnetars

NICER PROPOSAL: 02131     PI: Abigail Stevens

Comparing emission mechanisms of low-frequency QPOS in black holes and neutron stars

    The best laboratory for strong-field gravity is the inner 100s of kilometers around compact objects in low-mass X-ray binaries. The X-ray emission varies rapidly, notably in quasi-periodic oscillations (QPOs), which may be produced by general relativistic effects. Low-frequency QPOs from black holes and neutron stars share similar temporal and spectral properties, indicating that they have similar emission mechanisms, even though neutron stars have a surface and strong magnetic field in addition to general relativistic effects. With these proposed observations, we will use phase-resolved spectroscopy of two types of low-frequency QPOs to constrain and compare the emission mechanisms, to better understand accretion flows in the strong gravity regime.

NICER PROPOSAL: 03056     PI: Enoto

MAGNETIC ENERGY DISSIPATION OF MAGNETAR OUTBURSTS STUDIED VIA MULTI-WAVELENGTH FOLLOW-UP OBSERVATION

    Magnetar outburst is believed to be sporadic magnetic energy dissipation at interior or outside the strongest magnets in the Universe. However, the physics underlying this dissipation process is still unclear. The NICER's large eective area provides the best sensitivity to trace change of soft X-ray pulse proles during magnetar outbursts, which is crucial information of magnetic eld conguration and dissipation. For example, the NICER detected multiple pulse peaks from activated 4U 0142+61 as evidence for multipole surface elds. During the GO1 program, we successfully recorded single X-ray pulses for the rst time from the transient magnetar XTE J1810-197. Here, we propose 100-ks NICER follow-up ToO program for magnetar outbursts to investigate evolution of their soft X-ray pulse proles over their decays, compared with monitoring by hard X-ray and radio observatories.

NICER PROPOSAL: 03155     PI: Borghese

PROMPT NICER AND NUSTAR STUDY OF MAGNETAR OUTBURSTS

    Magnetars are a small class of neutron stars believed to be powered by their huge magnetic fields. They go through long stretches of quiescence, interrupted by periods of activity in the form of short X-ray bursts, giant flares and yearly-timescale X-ray outbursts. Multi-band observations of these transient events yield the largest amount of information on magnetars emission, allowing to test the theoretical models on a variety of phenomena and source states. We propose a joint NICER (50 ks) and NuSTAR (110 ks) ToO program (1 trigger of 4 observations over a few months) aimed at gathering new physical insights on magnetars surface and crust, magnetic field configuration and magnetospheres.

NICER PROPOSAL: 04060     PI: Enoto

MAGNETAR OUTBURSTS AS A CLUE FOR UNDERSTANDING MAGNETIC ENERGY DISSIPATION AND FAST RADIO BURSTS

    Magnetar X-ray outburst is sporadic magnetic energy dissipation of short bursts, giant flares, and persistent emission enhancement. The physics underlying this dissipation process is still unclear. Follow-up observations of transient magnetars with NICER have provided clues for this question, for example, detection of single X-ray pulses from the radio-loud magnetar XTE J1810-197, the burst forest from the Galactic FRB source SGR 1935+2154, and discoveries of Swift J1818.0-1607 and SGR 1830-0645. Prompt observations became much more critical after the discovery of the fast radio burst from the Galactic magnetar SGR 1935+2154 in 2020. Here we propose reserved NICER ToO observations of transient magnetar outbursts in soft X-rays coordinated with radio and hard X-ray simultaneous coverage.

NICER PROPOSAL: 04099     PI: Capitanio

THE FIRST X-RAY POLARIMETRY AND SPECTROSCOPY OBSERVATIONS OF A TRANSIENT BLACK HOLE X-RAY BINARY

   We propose joint NICER+NuSTARToO observations of a transient black hole X-ray binary (TB-HXB), to be taken in coordination with the Imaging X-ray Polarimetry Explorer (IXPE), a NASA mission that will launch in late 2021. The year one IXPE observing plan includes a ToO study of a TBHXB outburst, with one long (>  300 ks) exposure in the hard state and another in the soft state. We propose two 30 ks NICER+NuSTAR exposures during the hard state IXPE observation and one 20 ks NICER+NuSTAR exposure during the soft state IXPE observation. This will provide an unprecedented combination of X-ray polarimetry, high resolution broad-band spectroscopy and timing; yielding novel and unique information on the coronal geometry, the black hole spin and the role of jets in these sources.

NICER PROPOSAL: 04118     PI: Wang

RELATIVISTIC REFLECTION AND REVERBERATION MAPPING IN A BLACK HOLE BINARY

    Black hole astrophysics can be regarded as a fundamental tool in studying the accretion and ejection physics in the strongest gravity regime in the Universe. Reflection spectroscopy studies the time-averaged energy spectrum, providing constraints on properties in the accretion disk and the corona, but degeneracies take place in problems such as the truncation level of the disk. Reverberation mapping is a timing technique revealing the disk-corona geometry in the innermost regions, which could help break degeneracies. With recent cutting-edge developments in physical spectral-timing models, the main goal in this proposal is to find clues on the state transition mechanism, and the coupling between the disk, corona and jet in black hole binaries.

NICER PROPOSAL: 04152     PI: Sridhar

ILLUMINATING THE ACCRETION DISK-CORONAE OF BLACK HOLES DURING THEIR FADING PHASE WITH NICER

    We request 40 ks of NICER and 60 ks of NuSTAR time to trigger ToO observations to obtain high signal-to-noise spectra from the relatively under-explored fading phases of a non-failed outburst of transient black hole. Our principal goal is to track the evolving properties of the accretion-corona system viz., inner disk truncation, disk density, coronal geometry, and disk inclination as the outburst enters the returning hard-intermediate to quiescent-hard state. This will be achieved with a suite of relativistic reflection and reverberation models to investigate the implications of accretion physics on the origin of hysteresis in the hardness-intensity diagram.

NICER PROPOSAL: 04155     PI: Conners

THE PROPERTIES AND EVOLUTION OF ACCRETION DISKS IN BLACK HOLE BINARIES

    We propose simultaneous NICER and NuSTAR ToO observations of any one of the 19 listed transient black hole X-ray binaries. We request a total of 8 ks of NICER time, split into four 2 ks exposures, each simultaneous with a 20 ks NuSTAR exposure (total of 80 ks NuSTAR time), and each spaced about a week apart.

NICER PROPOSAL: 04163     PI: Homan

BROADBAND SPECTRA OF THE ULTRA-COMPACT X-RAY BINARY 4U 1820-30

    The modeling of neutron-star LMXB X-ray spectra often suffers from a lack of coverage at either the low or high energy end. NICER spectra offer great low-energy coverage, with the potential to obtain excellent constraints on the thermal components, including parameters such as the inner disk radius and size of the boundary layer. However, this still requires additional coverage above 10 keV. Here we request three 20 ks NICER/NuSTAR ToO observations of the ultra-compact neutron-star LMXB 4U 1820-30, a bright source with large spectral variations. The observations will be triggered to observe the source at select locations in its spectral range, with the goal of improving our understanding of the evolution of the various spectral components and the underlying changes in the accretion flow.

NICER PROPOSAL: 04181     PI: Mastroserio

MEASURING BLACK HOLE SPIN AND MASS THROUGH X-RAY REFLECTION SPECTRA AND REVERBERATION LAGS

    X-ray reflection in accreting black holes probes the inner region of the accretion disc, and proper modelling of the spectral and timing properties of this emission enables measurement of the black hole mass and spin. The unique combination of NICER s soft and NuSTAR's hard coverage provide the broad bandpass, high count rates and energy and timing resolution required to constrain models of the time averaged energy spectrum and the reverberation lag energy spectrum on different timescales. We propose to observe any black hole transient, known or unknown, exceeding 30 mCrab during the bright hard state for 30 ks with simultaneous NICER and NuSTAR observation in order to access unprecedented characterisation of black hole spin and mass.

NICER PROPOSAL: 05001     PI: Fiamma Capitanio

THE FIRST X-RAY POLARIMETRY AND SPECTROSCOPY OBSERVATIONS OF TRANSIENT BLACK HOLE X-RAY BINARIES

   We propose 3 joint nicer+nustar ToOs observations of transient black hole X-ray binaries to be taken in coordination with IXPE, a NASA-ASI mission that will be launched on Dec,9,2021.The first year IXPE observing plan includes up to 3ToO of TBHXB in outburst.Each ToO includes one long exposure in the hard state and another in the soft(> 300ks). We propose(for each ToO) two 30ks nicer+nustar~exposures during the hard state IXPE pointing and one 20 ks nicer+nustar~exposure during the soft state IXPE pointing. This will provide an unprecedented combination of X-ray polarimetry, high resolution spectroscopy and timing; yielding unique information on coronal geometry, blackhole spin and role of the jets.

NICER PROPOSAL: 05065     PI: Riley Connors

NICER CONSTRAINTS ON BLACK HOLE BINARY ACCRETION PHYSICS

   A number of open questions persist in our understanding of the accretion flow properties of outbursting black hole binaries: principally the inner disk radius location, the disk density, iron abundance and inclination, and black hole spin. The superior spectral/timing resolution of NICER, and the high energy coverage provided by NuSTAR, present a golden opportunity to get constraints on these key quantities. We propose simultaneous NICER and NuSTAR ToO observations of any one of the 18 listed transient black hole X-ray binaries. We request a total of 8 ks of NICER time, split into four 2ks exposures, each simultaneous with a 20 ks NuSTAR exposure (total of 80 ks NuSTAR time), to be spaced within a week of one another.

NICER PROPOSAL: 05076     PI: Teruaki Enoto

MAGNETAR OUTBURSTS AS A CLUE FOR UNDERSTANDING MAGNETIC ENERGY DISSIPATION AND FAST RADIO BURSTS

   Magnetar X-ray outburst is sporadic dissipation of magnetic energy via short bursts, giant flares, and persistent emission enhancement. The physics underlying this process is still an open question. Follow-up NICER observations of transient magnetars have provided clues for this question, as shown by recent successful NICER ToO programs: the radio-loud XTE J1810-197, the Galactic fast radio burst (FRB) source SGR 1935+2154, high-B pulsar / magnetar Swift J1818.0-1607, a new source SGR 1830-0645, and the long-term active Swift J1555.2-5402. Prompt X-ray observation is becoming more and more critical after discovering the FRBs from the Galactic magnetar SGR~1935+2154 in 2020. Here we propose NICER ToO observations of magnetar outbursts in soft X-rays.

NICER PROPOSAL: 05079     PI: Alice Borghese

FOLLOWING A NEW MAGNETAR OUTBURST WITH NICER AND NUSTAR

   Isolated neutron stars powered by the instabilities and decay of their huge magnetic field, magnetars are characterized by a distinctive high-energy flaring phenomenology: short bursts of X-/gamma-rays, often accompanied by enhancements of the persistent X-ray luminosity, referred to as outbursts. Magnetar-like activity was discovered from isolated neutron stars with a broad range of magnetic field strengths. Moreover, the recent detection of a FRB-like burst from a Galactic magnetar has strengthened the belief that at least a sub-group of FRBs can be powered by magnetars. Here, we propose to follow two new outbursts from a known or a new source with NICER and NuSTAR to gather new physical insights on magnetar surface, field configuration and magnetosphere.

NICER PROPOSAL: 05099     PI: Federico Garcia

LF-QPOS IN BH-LMXBS: EXPLORING THE ROLE OF COMPTONISATION WITH NICER AND NUSTAR

   BH LMXBs show prominent low-frequency QPOs in their power-density spectra, with fractional amplitudes that increase with energy and complex lag spectra. The question about their physical origin remains unanswered. Quasi-simultaneous observations with NICER and NuSTAR in the soft and hard X-ray bands will allow us to probe the radiative mechanism that modulates the energy-dependent timing properties of these LF QPOs. This information is crucial to unveil the physical mechanism that produces these QPOs, both dynamically, either connected to Lense-Thirring Precession or through instabilities in the hot accretion flow, and radiatively, via Compton amplification in the corona where the observed spectral-timing properties are imprinted onto the X-ray emission.

NICER PROPOSAL: 05106     PI: Filioop D Ammando

NICER AND NUSTAR FOLLOW-UP OBSERVATIONS OF GAMMA-RAY FLARING BLAZARS

   There are many open questions related to the nature of the high-energy emission and the physics of jets which need to be addressed, such as the radiative processes producing the high-energy emission, the parameters of the emitting region(s) and the jet composition. Radiative models need to be tested against MWL simultaneous SED. We propose to trigger 1 ToO observation with NICER and NuSTAR if gamma-ray flaring activity from a blazar is detected by Fermi-LAT and a high X-ray flux has been confirmed by a rapid Swift follow-up observation. The goal is the study of the broadband SED and MWL light curves for putting constraints on the physics of relativistic jets from super-massive black holes, and on the acceleration and radiation mechanisms at work in such extreme environments.

NICER PROPOSAL: 05118     PI: Jingyi Wang

RELATIVISTIC REFLECTION AND REVERBERATION MAPPING IN A BLACK HOLE BINARY

   Black hole astrophysics can be regarded as a fundamental tool in studying the accretion and ejection physics in the strongest gravity regime in the Universe. Reflection spectroscopy studies the time-averaged flux-energy spectrum, providing constraints on properties in the accretion disk and the corona, but degeneracies take place in problems such as the truncation level of the disk. Reverberation mapping is a timing technique revealing the disk-corona geometry in the innermost regions, which could help break degeneracies. With recent cutting-edge developments in physical spectral-timing models, the main goal in this proposal is to find clues on the state transition mechanism, and the coupling between the disk, corona and jet in black hole binaries.

NICER PROPOSAL: 05131     PI: Joseph Neilsen

THE OBSCURED STATE OF GRS 1915+105

   GRS 1915+105 is a black hole binary known for its unique variability, strong winds, jets, and BH spin. After 20+ years in outburst, NICER detected a huge change in this iconic source: the X-ray flux dropped by 100x! Spectra suggest a large obscuring shroud, but what is this obscuring gas? The "obscured" state is highly variable, with flares that reveal strong winds and highly-ionized absorption. We propose to study its long-term evolution with 35 weekly exposures of 2.7 ks in Cycle 4. NICER is the only mission capable of frequently monitoring this important new state. We also request a 25 ks NuSTAR ToO to constrain scattering and wind photoionziation. These observations will also grow a NICER legacy archive of obscured variability in GRS 1915+105.

NICER PROPOSAL: 05140     PI: Guglielmo Mastroserio

MEASURING BLACK HOLE SPIN AND MASS THROUGH X-RAY REFLECTION SPECTRA AND REVERBERATION LAGS

   X-ray reflection in accreting black holes probes the inner region of the accretion disc, and proper modelling of the spectral and timing properties of this emission enables measurement of the black hole mass and spin. The unique combination of NICER s soft and NuSTARs hard coverage provide the broad bandpass, high count rates and energy and timing resolution required to constrain models of the time averaged energy spectrum and the reverberation lag energy spectrum on different timescales. We propose to observe any black hole transient, known or unknown, exceeding 30 mCrab during the bright hard state for 30 ks with simultaneous NICER and NuSTAR observation in order to access unprecedented characterisation of black hole spin and mass.

NICER PROPOSAL: 05172     PI: Javier Garcia

THE PROPERTIES AND EVOLUTION OF ACCRETION DISKS IN BLACK HOLE BINARIES

   We request 10ks of NICER and 100ks of NuSTAR time to trigger Target of Opportunity (ToO) observations of a new or known black hole transient during the end of the outburst, after the source has entered the low-hard state. We aim to obtain high signal-to-noise data during this fainter phase in order to measure the level of disk truncation using X-ray reflection spectroscopy. We will also provide measurements for the disk inclination, ionization and iron abundance. These observations will provide crucial constraints to support a large-scale data analysis program for these sources.

NICER PROPOSAL: 06039     PI: Teruaki Enoto

MAGNETAR OUTBURSTS AS A CLUE FOR UNDERSTANDING MAGNETIC ENERGY DISSIPATION AND FAST RADIO BURSTS

   A magnetar X-ray outburst is a sporadic dissipation of magnetic energy via short bursts, giant flares, and persistent emission enhancement. The physics underlying this process is still an open question. Follow-up NICER observations of transient magnetars have provided clues for this question, as shown by recent successful NICER ToO programs: the radio-loud XTE J1810-197, the Galactic fast radio burst (FRB) source SGR 1935+2154, high-B pulsar resembles Swift J1818.0-1607, a new source SGR 1830-0645, and the long-term active Swift J1555.2-5402. Prompt X-ray observation is becoming more and more critical after discovering the FRBs from the Galactic magnetar SGR 1935+2154 in 2020. Here we propose NICER ToO observations of magnetar outbursts in soft X-rays coordinated with radio observations (e.g., DSN) and hard X-ray (NuSTAR) measurements. This program provides a clue for understanding the energy dissipation process of magnetars and the FRB mystery.

NICER PROPOSAL: 06057     PI: Fiamma Capitanio

THE FIRST X-RAY POLARIMETRY AND SPECTROSCOPY OBSERVATIONS OF TRANSIENT BLACK HOLE X-RAY BINARIES

   We propose 3 joint NICER+NuSTAR ToOs observations of transient black hole X-ray binaries (TBHXBs), to be taken in coordination with the Imaging X-ray Polarimetry Explorer(IXPE), a NASA-ASI mission launched on December 9, 2021. As the first year, the second year IXPE observing plan includes 3 ToOs of TBHXB in outburst. Each ToO includes one long (# 300 ks) exposure in the hard state and another in the soft state. In fact, until now IXPE has not had an opportunity to observe a complete outburst of a TBHXB because of the lack of bright standard outbursts occurred within the visibility windows. We propose (for each ToO) two 30 ks NICER+NuSTAR exposures during the hard state IXPE observation and one 20 ks NICER+NuSTAR exposure during the soft state IXPE observation. This will provide, as demonstrated by the IXPE first year results, an unprecedented combination of X-ray polarimetry, high resolution broad-band spectroscopy and timing; yielding novel and unique information on the coronal geometry, the black hole spin and the role of jets in these sources.

NICER PROPOSAL: 06078     PI: Yuhan Yao

NICER+NUSTSR OBSERVATIONS OF TIDAL DISRUPTION EVENTS: OPENING A NEW CHAPTER IN BLACK HOLE SUPER-EDDINGTON ACCRETION

   Tidal disruption events (TDEs) offer great opportunities to study the physics of black hole accretion in the super-Eddington regime. The field of X-ray TDE study is still so young such that each object reveals new and unexpected phenomena. We propose high-cadence NICER monitoring on two X-ray bright TDEs to address: (1) how often and on what timescales does corona formation happen; (2) does the thick-to-thin disk transition happen in TDEs other than AT2021ehb; (3) what is the fraction of TDEs that launch on-axis relativistic jets. We also ask for one epoch of NuSTAR observation for one TDE that is sufficiently bright in the hard X-ray. The NICER+NuSTAR spectrum will allow us to study the geometry of the super-Eddington accreting inflow and the powering of the outflow/jet.

NICER PROPOSAL: 06113     PI: Adam Ingram

THE FIRST X-RAY POLARIMETRY AND SPECTROSCOPY OBSERVATIONS OF GRS 1915+105

   We propose three 30ks NICER exposures contemporaneous with three 20ks NuSTAR exposures of the black hole (BH) X-ray binary (XRB) GRS 1915+105, to be taken during a six-day long Imaging X-ray Polarimetry Explorer (IXPE) target of opportunity (ToO) observation of the source. The IXPE ToO will be triggered when the source resumes behaviour for which a detection of X-ray polarisation can reasonably be expected, and we will trigger this companion program in response. The high spectral resolution and broad band pass provided by NICER+NuSTAR coverage will be vital for breaking degeneracies in polarization models.

NICER PROPOSAL: 06141     PI: Jingyi Wang

RELATIVISTIC REFLECTION AND REVERBERATION MAPPING IN A BLACK HOLE BINARY

   Black hole astrophysics can be regarded as a fundamental tool in studying the accretion and ejection physics in the strongest gravity regime in the Universe. Reflection spectroscopy studies the time-averaged flux-energy spectrum, providing constraints on properties in the accretion disk and the corona, but degeneracies take place in problems such as the truncation level of the disk. Reverberation mapping is a timing technique revealing the disk-corona geometry in the innermost regions, which could help break degeneracies. With recent cutting-edge developments in physical spectral-timing models, the main goal in this proposal is to find clues on the state transition mechanism, and the coupling between the disk, corona and jet in black hole binaries.

NICER PROPOSAL: 06143     PI: James Steiner

NICER, NUSTAR, AND IXPE SPECTRO-POLARIMETRIC CONSTRAINTS ON THE SOFT STATE OF CYGNUS X-1

   A groundbreaking recent study of the hard state of Cygnus X-1 with IXPE, NICER and NuSTAR provided the first robust soft X-ray polarimetric detection of an accreting black hole. The spectro-polarimetric results yielded unprecedented constraints on the jet, corona, and accretion disk geometry. This was anchored in the broadband X-ray spectral data from NICER and NuSTAR being modeled in conjunction with IXPE's polarimetric data. If Cyg X-1 transitions to its soft state, an approved 600ks IXPE campaign will be triggered. To achieve equivalent constraints on the soft state's accretion geometry as with the hard state campaign, we request an accompanying 5ks per day for 12 days with NICER, and 3 x 20ks observations with NuSTAR during the IXPE run, 60ks apiece for NICER and NuSTAR.

NICER PROPOSAL: 06197     PI: Joseph Neilsen

THE OBSCURED STATE OF GRS 1915+105

   GRS 1915+105 is a black hole binary known for its unique variability, strong winds, jets, and BH spin. After 20+ years in outburst, NICER detected a huge change in this iconic source: the X-ray flux dropped by 100x! Spectra suggest a large obscuring shroud, but what is this obscuring gas? The "obscured" state is highly variable, with flares that reveal strong winds and highly-ionized absorption. We propose to study its long-term evolution with 35 weekly exposures of 2.7 ks in Cycle 5. NICER is the only mission capable of frequently monitoring this important new state. We also request a 25 ks NuSTAR ToO to constrain scattering and wind photoionziation. These observations will also grow a NICER legacy archive of obscured variability in GRS 1915+105.

NICER PROPOSAL: 06203     PI: PAUL DRAGHIS

TRACKING TWO FUTURE BLACK HOLE OUTBURST WITH NICER AND NUSTAR

   We propose monitoring two future, previously unobserved BH transients with fluxes above 50 mCrab by dedicating 45 ks of NICER time over 15 observations spread throughout each of the two outbursts. By pairing a 30 ks NuSTAR observation with a NICER observation, we can obtain a precise spin measurement for each BH. The NICER monitoring campaign will probe the effects of the evolution of the outburst on the spin measurement. This project will test the assumptions and hypotheses of BH spin measurements in X-ray binaries, and will serve as a legacy for NICER and NuSTAR. In the future, this treatment will maximize the number of new BHs observed and, together with gravitational wave observations, will allow developing a unifying understanding of stellar-mass black hole formation and evolution.

NICER PROPOSAL: 07041     PI: Alice Borghese

CATCHING A NEW MAGNETAR OUTBURST WITH NICER AND NUSTAR

   Isolated neutron stars powered by the instabilities and decay of their huge magnetic field, magnetars are characterized by a distinctive high-energy flaring phenomenology: short bursts of X-/gamma-rays, often accompanied by enhancements of the persistent X-ray luminosity, referred to as outbursts. Magnetar-like activity was discovered from isolated neutron stars with a broad range of magnetic field strengths. Moreover, the recent detection of a FRB-like burst from a Galactic magnetar has strengthened the belief that at least a sub-group of FRBs can be powered by magnetars. Here, we propose to follow two new magnetar-like outbursts from a known or a new source with NICER and NuSTAR to gather new physical insights on magnetar surface, field configuration and magnetosphere.

NICER PROPOSAL: 07082     PI: Teruaki Enoto

PROMPT AND HIGH-CADENCE X-RAY FOLLOW-UP OF TRANSIENT MAGNETARS FOR UNDERSTANDING THE MAGNETAR-FRB CONNECTION

   The phenomenon of Fast Radio Bursts (FRBs) is one of the most tantalizing mysteries in astronomy. However, the mechanism of how magnetars initiate FRBs is poorly understood. In October 2022, this magnetar exhibited FRBs and short X-ray bursts again. During this magnetar outburst, ToO observations and time series analyses with NICER and NuSTAR revealed a pair of glitches that occurred before and after FRBs, associated with a burst forest, providing valuable insights into this mystery. Hence, this proposal aims to prompt and high-cadence X-ray monitoring during an outburst of transient magnetars to investigate their X-ray behaviours and to search for glitches similar to those of SGR 1935+2154 and also for FRB-associated X-ray bursts.

NICER PROPOSAL: 07126     PI: Gugleilmo Mastroserio

MEASURING BLACK HOLE SPIN AND MASS THROUGH X-RAY REFLECTION AND REVERBERATION LAGS

   X-ray reflection in accreting black holes probes the inner region of the accretion disc, and proper modelling of the spectral and timing properties of this emission enables measurement of the black hole mass and spin. The unique combination of NICER s soft and NuSTARs hard coverage provides the broad bandpass, high count rates and energy and timing resolution required to constrain models of the time-averaged energy spectrum and the reverberation lag energy spectrum on different timescales. We propose to observe any black hole transient, known or unknown, exceeding 50 mCrab during the bright hard state for 50 ks with simultaneous NICER and NuSTAR observation in order to access unprecedented characterization of black hole spin and mass.

NICER PROPOSAL: 07141     PI: Muryel Guolo

NICER+NUSTAR OBSERVATIONS OF TIDAL DISRUPTION EVENTS: PROBING THE REAL-TIME FORMATION OF JETS AND X-RAY CORONAE

   Tidal disruption events (TDEs) offer excellent opportunities to study the physics of black hole accretion, including the real-time formation of jets and X-ray coronae. The field of X-ray TDE study is still so young that each object reveals new and unexpected phenomena. We propose high-cadence NICER monitoring on one X-ray bright TDE showing hard X-ray emission: i) if triggered in a relativistic/jetted TDE, we aim to probe the physical properties of nascent jet; ii) if triggered in a non-relativistic TDE, we aim to probe the accretion state transitions, the real-time formation of a hard X-ray corona and search for reflection features akin to AGN. The NICER+NuSTAR broad-band spectrum will allow us to study the geometry of the super-Eddington accreting inflow and the powering of the outflow

NICER PROPOSAL: 07164     PI: Keigo Fukumura

BROADER PERSPECTIVES OF ACCRETION DISK AND MULTI-ION UFOS IN MAXI J1810-222

   We propose a multi-epoch, joint ToO program of BH XRB, MAXI J1810-222, to study a physical link between multi-ion ultra-fast outflows (UFOs) and yet-to-be explored reflection features with NICER (for soft X-ray UFOs) and NuSTAR (for Fe K and reflection) during an outburst. Being triggered by MAXI/GSC daily monitoring, we aim to unpack a concealed nature of powerful disk winds and disk reflection by systematically modeling and analyzing multi-epoch NICER+NuSTAR spectra. Our study is conducted by a physically-motivated wind model in the context of magnetic-driving in conjunction with state-of-the-art reflection spectroscopy to further understand a broadband X-ray phenomenology of this XRB for which NICER (30ks x3) and NuSTAR (20ks x3) play indispensable roles.

NICER PROPOSAL: 07170     PI: Edward Nathan

RESOLVING A QUASI PERIODIC OSCILLATION IN GRS1915+105 UPON ITS RETURN TO AN UNOBSCURED STATE

   Before 2018, the black hole X-ray binary GRS1915+105 was extremely bright in X-rays, and at times showed powerful type-C quasi-periodic oscillations (QPOs). We propose a joint 30 ks NICER and 30 ks NuSTAR observation of a QPO in GRS1915+105 if the source leaves its current X-ray obscured state. We will apply state-of-the-art phase resolving techniques to test for the presence of a precessing corona via its effects on the flux reflected from the accretion disc.

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