High count rate effects in event processing for XRISM/Resolve x-ray microcalorimeter: II. Energy scale and resolution in orbit

TL;DR

This paper investigates high count rate effects on energy scale and resolution in XRISM/Resolve microcalorimeter observations of bright X-ray sources, providing insights for data analysis and future mission planning.

Contribution

It presents an analysis of high count rate effects on energy scale and resolution, including correction methods, using real observational data from the Crab Nebula and GX 13+1.

Findings

Energy scale shifts negatively at high count rates.

Energy resolution degrades with neighboring pixel count rates and can be corrected.

High count rate effects impact velocity measurements in bright source observations.

Abstract

The Resolve instrument on the X-ray Imaging and Spectroscopy Mission (XRISM) uses a 36-pixel microcalorimeter designed to deliver high-resolution, non-dispersive X-ray spectroscopy. Although it is optimized for extended sources with low count rates, Resolve observations of bright point sources are still able to provide unique insights into the physics of these objects, as long as high count rate effects are addressed in the analysis. These effects include {the loss of exposure time for each pixel}, change on the energy scale, and change on the energy resolution. To investigate these effects under realistic observational conditions, we observed the bright X-ray source, the Crab Nebula, with XRISM at several offset positions with respect to the Resolve field of view and with continuous illumination from {$^{55}$Fe sources} on the filter wheel. For the spectral analysis, we excluded data where exposure time loss was too significant to ensure reliable spectral statistics. The energy scale at 6 keV shows a slight negative shift in the high-count-rate regime. The energy resolution at 6 keV worsens as the count rate in electrically neighboring pixels increases, but can be restored by applying a nearest-neighbor coincidence cut (``cross-talk cut''). We examined how these effects influence the observation of bright point sources, using GX 13+1 as a test case, and identified an eV-scale energy offset at 6 keV between the inner (brighter) and outer (fainter) pixels. Users who seek to analyze velocity structures on the order of tens of km~s$^{-1}$ should account for such high count rate effects. These findings will aid in the interpretation of Resolve data from bright sources and provide valuable considerations for designing and planning for future microcalorimeter missions.