Pile-up simulator for XRISM/Xtend onboard the X-ray Imaging and Spectroscopy Mission (XRISM)

Abstract: In X-ray astronomy, most observatories utilize multi-pixel photon-counting devices. When a photon counting device observes a bright source, we face an unavoidable problem called pile-up. Pile-up leads to mistakes in the observational properties of the source, mainly an apparent decrease in the X-ray flux. X-Ray Imaging and Spectroscopy Mission (XRISM) has two X-ray telescopes, one of which is Xtend, a CCD camera with a wide field-of-view (FOV) of 38 arcmin square. Xtend has three operating modes: full window mode with a frame exposure of ~4 s, 1/8 window mode with ~0.5s and reduced FOV, and 1/8 window mode with burst option, whose frame exposure is reduced to ~0.06 s. Observers need to select the operating mode according to their target fluxes. We develop the pile-up simulator for Xtend to provide a quantitative assessment of pile-up according to the fluxes, spectra, and shapes of X-ray sources. We derived the 10% pile-up limits for a point source of 7.8, 66.2, and 447.9 counts/s for full window, 1/8 window, and 1/8 window mode with burst option, respectively, by assuming the Crab spectrum. We present further simulations for a diffuse source and monochromatic spectra.