Pulse Parameters Optimization of the Modulated X-Ray Sources for the Resolve Microcalorimeter Spectrometer on XRISM

Resolve is an X-ray microcalorimeter spectrometer on the X-Ray Imaging and Spectroscopy Mission (XRISM) to be launched in Japanese fiscal year 2022. Resolve is required to achieve an energy resolution of 7 eV at FWHM at 6 keV. To satisfy this requirement, it is necessary to correct the in-orbit gain drift. For this purpose, Resolve is equipped with multiple gain tracking calibration sources, including the modulated X-ray sources (MXS). The MXS will be operated in a pulsed mode, in which calibration X-rays illuminating the detector array are emitted at a duty cycle of similar to 1%. The low duty cycle allows us to monitor the gain drift with a small loss of the observing efficiency. However, the use of the MXS has drawbacks such as increase in the instrumental background due to exponentially decaying afterglow emission following each MXS pulse and the loss of throughput due to changes in the event-grade branching ratio. To minimize these effects, an optimization of the MXS operating parameters is needed. Based on the results of the MXS component-level tests, we established an analytical model that describes the MXS pulse and afterglow count rates. We obtained the optimal pulse parameters for various gain tracking intervals and estimated the effects of using the MXS on observation data. We further studied the trade-off between these effects and resolution degradation using the actual in-orbit drift observed with the Soft X-ray Spectrometer on the Hitomi satellite. Our study forms the basis of strategies for the in-orbit gain drift correction of Resolve.