Proceedings Article | 21 August 2024
KEYWORDS: Monte Carlo methods, X-rays, Imaging spectroscopy, X-ray imaging, X-ray telescopes, Photonic devices, Charge-coupled devices, X-ray astronomy
In X-ray astronomy, most observatories utilize multi-pixel photon-counting devices to obtain spatial, timing, and energy information of X-ray photons radiated by astronomical objects. When a photon-counting device observe 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 telescopes utilizing CCDs as multi-pixel photon-counting devices for imaging and spectroscopy, such as ASCA, Chandra, XMM-Newton, and Suzaku, have operating modes to avoid pile-up for bright sources. 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 sec, 1/8 window mode with ∼0.5 sec 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 and scientific purposes. 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. The simulator is based on the Monte-Carlo simulation utilizing the GEANT4 framework. We derived the 10% pile-up limits for a point source of 7.8, 66.2, and 447.9 counts s−1 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.
