X-ray transmission calibration of the gate valve for the X-ray astronomy satellite XRISM

TL;DR

This paper details the ground calibration and modeling of the X-ray transmission of the gate valve for the XRISM satellite's Resolve instrument, ensuring accurate initial observations through detailed measurements and physical modeling.

Contribution

It provides the first comprehensive calibration and physical modeling of the XRISM gate valve's X-ray transmission, including effects of Bragg diffraction in Be.

Findings

Transmission measurements meet calibration requirements

Model accurately reproduces observed transmission features

Improved spectral fitting of Crab nebula data

Abstract

\textit{Resolve} onboard the X-ray satellite XRISM is a cryogenic instrument with an X-ray microcalorimeter in a Dewar. A lid partially transparent to X-rays (called gate valve, or GV) is installed at the top of the Dewar along the optical axis. Because observations will be made through the GV for the first few months, the X-ray transmission calibration of the GV is crucial for initial scientific outcomes. We present the results of our ground calibration campaign of the GV, which is composed of a Be window and a stainless steel mesh. For the stainless steel mesh, we measured its transmission using the X-ray beamline at ISAS. For the Be window, we used synchrotron facilities to measure the transmission and modeled the data with (i) photoelectric absorption and incoherent scattering of Be, (ii) photoelectric absorption of contaminants, and (iii) coherent scattering of Be changing at specific energies. We discuss the physical interpretation of the transmission discontinuity caused by the Bragg diffraction in poly-crystal Be, which we incorporated into our transmission phenomenological model. We present the X-ray diffraction measurement on the sample to support our interpretation. The measurements and the constructed model meet the calibration requirements of the GV. We also performed a spectral fitting of the Crab nebula observed with Hitomi SXS and confirmed improvements of the model parameters.