Conclusion
With the current NuSTAR calibration we find deviations from the time-dependent gain stored in the CALDB file of substantially less than 1 NuSTAR channel (40 eV) over the three year baseline currently available in the Kepler data set. We will continue to monitor Kepler once per year to monitor any changes in the Offset that may be required.
Over the ~4.5 year baseline from mid-2012 to early 2017 we find small residual deviations in the Slope parameter. At 80 keV, these deviations correspond to a shift in a line centroid of roughly 40 eV, or one NuSTAR channel. However, the uncertainties in the Slope measurements are large enough that we do not consider additional corrections to the NuSTAR gain required at this time.
We consider these results a confirmation that we have adequately characterized the time-dependence of the gain of the NuSTAR detectors as a function of time.
Appendix: History of Gain-Related CALDB Files: