Gain Monitoring

As all of the lines in Kepler are at relatively low energy, we cannot use these observations to constrain the Slope parameter. We can, however, use  high-energy background lines to accomplish this. We integrate over all data where NuSTAR is pointing at the Earth (know as "MODE02" in NuSTARDAS parlance) and compare the year-over-year spectra. We use the data from 100 -- 150 keV, which are relatively easy to interpret as the contributions mainly come from the internal background continuum component as well as lines from the calibration source at 105, 122, and 144 keV.
The internal continuum is modeled as a broken power-law and, as we expect this to derive from down-scattered components in the instrument, we use a diagonal RMF (rather than the NuSTAR CZT RMFs that are appropriate for incident photons). Since the continuum model is dependent on the space weather conditions (which may vary year-over-year) we allow the power-law indices above and below the break energy (fixed to 124 keV) as well as the normalization of the broken power law to vary between the two year-long spectra. The only other free parameter is the Slope, implemented here by Xspec's "gainfit" formalism with the Offset parameter frozen to zero. A comparison of the 2012 and 2017 background epochs are shown below after best-fit gain shift have been applied. We do not find any evidence for an additional gain shift, verifying our assumption that the gain drift is linear with time.