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

TL;DR

The paper addresses pile-up in XRISM Xtend CCD observations and introduces a Monte Carlo–based pile-up simulator built on Geant4 within the ComptonSoft framework to quantify pile-up as a function of flux, spectrum, and source morphology. By calibrating to Xtend/SXI specifications, the tool derives mode-dependent $10\%$ pile-up limits for a Crab-like spectrum and extends analyses to monochromatic and diffuse sources. Key findings include mode-specific pile-up thresholds of $7.8$, $66.2$, and $447.9$ counts s$^{-1}$ for Full Window, 1/8 Window, and 1/8 Window with burst, along with spectral hardening and grade migration effects at high flux. The simulator provides actionable guidance for Xtend observing strategies and is publicly documented in the XRISM proposers' guide, enhancing reliable flux and spectral measurements for bright X-ray sources.

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.

Figures (6)

• Figure 1: Simulated spectra by the pile-up simulator by assuming a point source with the Crab spectrum with fluxes of $10^{-6}$--$1$ Crab. The full window mode is simulated in this figure. High input fluxes result in the high pile-up effect, by which fractions of photons are lost in the simulated spectra (see figure \ref{['fig:point_crab']} for the quantitative evaluation of the pile-up).
• Figure 2: Pile-up fraction for each mode about intensity. Black, red, and Green plots correspond to full window mode, 1/8 window mode, and 1/8 window mode with 0.1 sec burst option, respectively. The blue dashed line denotes the 10% pile-up limit.
• Figure 3: Photon indices ($\Gamma$) of the simulated spectra about the input flux for the full-window-mode observation of the Crab-like spectrum. The higher flux induces the heavier distortion of the spectral shape. The dashed line denotes the input photon index of 2.1.
• Figure 4: Normalized Grade branching ratios for the simulated events. Black, red, and blue lines denote 1 $\mu$Crab, 10 mCrab, and 1 Crab, respectively. Good grades are 0, 2, 3, 4, 6, and 8, from which X-ray spectra are extracted. Events with other grades are regarded as non-X-ray events.
• Figure 5: Pile-up fraction for each mode about the PSF core exclusion radius in units of pixels. The top left, top right, and bottom left panels correspond to full window mode, 1/8 window mode, and 1/8 window mode with 0.1 sec burst option, respectively. For each panel, the blue dashed lines denote the 10% pile-up limit.