What Is The MLI correction?

In roughly October 2017, the thermal blanketing on the rear of the FPMA telescope suffered a tear. This results in an enhanced soft X-ray throughput for FPMA compared with FPMB after this occurred. The NuSTAR calibration team updated the effective area of the optics to compensate for the loss of X-ray attenuation in the MLI in the NuSTAR 20200429 CALDB release (and the subsequent CALDB and NuSTARDAS releases) . The MLI correction accounts for the additional degradation in the MLI as a funciton of itme.

In addition, there is evidence for short periods of time when the optics became colder, indicating a further loss of MLI. However, these periods of time are limited to relatively short periods of time in ~2017/2018.

Extensive documentation of this issue may be found in Madsen, K.K., et al. (2020)   arXiv:2005.00569

For these observations, the model of the MLI behavior may not fully track the performance of the FPMA optic. Systematic errors are expected to be on the order of a few percent, so that this is primarily visible for high signal-to-noise observations (here meaning bright X-ray binaries with high count rates) as a deviation between FPMA an FPMB at low (E<10 keV) energies. A temperature-dependent MLI correction was introduced in the August 2020 NuSTARDAS v2.0.0 release (with HEASoft 6.28) that used a particular thermistor to determine if additional MLI was required. The number of observations where the temperature-dependent correction is required was small, so to avoid confusion the default behavior of nuproducts was changed in the July 2021 NuSTARDAS v.2.1.1 release to not apply the temperature dependent correction. In late 2022, the thermistor used to drive this temperature-dependent correction is no longer providing accurate thermal readings. The temperature-dependent correction should not be used after December 2022 for any observation. An update to the NuSTARDAS is in progress that will prevent the user from accidentally applying the temperature-dependent correction outside of the intended time range.

We do note that there may be a subset of bright sources with the MLI correction not be accurate. This especially true after 2022, where we no longer have accurate temperature information to determine if an additional MLI correction is required before inspecting the data. For these observations we provide an Xspec mtable model to be used for those observations. Care must be taken in applying this mtable as it may potentially skew the results if not used properly. When in doubt, please contact the NuSTAR SOC for guidance on the use of this mtable.

This mtable model should not be used to force a model into agreement, but only if significant differences are observed between FPMA and FPMB.

The table below lists the observation sequenceID's where there MLI fraction is below 0.9 and where we recommend the use of the mtable model (or the temperature-dependent correction).

How Is It Used?

The MLI mtable correction is made available for pathological cases when there is a substantial difference between FPMA and FPMB. If these steps are not followed, and the mtable parameters and the constants between A and B are allowed to fit freely, incorrect estimates of the continuum and line shapes may occur.

1. First ignore FPMA between 3 and 7 keV, when doing your original fits. Make sure the model includes a constant offset between FPMA and FPMB. Find the value of the constant and freeze it.

2. Notice FPMA between 3 and 7 keV and add the mtable to the model. Set the fraction value of the MLI to 1.0 for FPMB and freeze it. Allow only the MLI fraction of FPMA to fit.

3. Please note: The range of MLI fraction should now lie between 0.9 and 1.0, and if the range is outside that, something else may be wrong.

Note: In some instances, the MLI correction may be too aggressive. This may be seen as the flux of FPMA being lower than FPMB between 3-5 keV, and the reason for this is that the change in MLI is gradual whereas the corrections are a step function.
In these cases one can revert to the old FPMA ARF. This can be done by running the keyword   inarffile=   in nuproducts and point it to   caldb/data/nustar/fpm/bcf/arf/nuA20100101v006.arf

Available MLI models:

nuMLIv1.mod


    Generated May 01, 2020

NuSTAR project website:   nustar.caltech.edu

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