In particular, requests for observations of the following types of sources and events require special justification describing why the requested observation type could not have been proposed as a GO target:
Known or candidate black hole systems
Known or candidate pulsar or magnetar systems
Candidate counterparts of gravitational wave or astrophysical neutrino detections
Nova or Supernova events
Luminosity variations from known or candidate AGN (including blazars and Tidal Disruption Events)
Observations of the Sun or other Solar system objects
Observations of events with specific time constraints
Observations coordinated with other observatories that could have been proposed for to joint GO programs
As a specific example, binary systems where periastron intervals are known can be proposed to the GO program, with observations triggered from monitoring of activity by other observatories. Observations that are simultaneous with ground or space based activities can also be proposed to the GO program, even if the exact dates for the coordinated observations are not known at the time of the proposal.
Please review the NuSTAR schedules for details of upcoming observations and the table of approved ToO programs from the NuSTAR GO program and joint GO programs with Chandra, XMM-Newton, INTEGRAL, and the Neil Gehrels Swift Observatory.