Simulation study of pulse height difference between pixel patterns of X-ray CCDs onboard the XRISM satellite

We have developed a soft X-ray telescope system Xtend onboard Japan's new X-ray astronomical satellite XRISM. Xtend employs X-ray CCDs, which have basically the same design as those used in the previous X-ray astronomical satellite Hitomi. Monochromatic X-ray line spectra of the CCDs onboard Hitomi showed offsets of centroids between different pixel patterns or different good grades; the spectral centroids of charge-sharing events are higher than those of single-pixel events. The spectral offset, which we call “Goffset”, can cause large uncertainties in X-ray energy determination accuracy. In this paper, confirming the CCDs onboard XRISM also have Goffset, we performed a simulation study that takes into account the two factors; charge sharing and readout noise. Goffset of the flight model CCD is successfully reproduced by the simulation with a certain readout noise. We investigate how charge sharing and readout noise cause Goffset based on the simulation results.