In-flight measurements of the optical axis and stray light of x-ray mirror assembly (XMA) on XRISM

Abstract

The X-ray Mirror Assemblies (XMAs) on board the X-ray Imaging and Spectroscopy Mission (XRISM) underwent dedicated in-flight calibration during the early phase of the mission to ensure high-precision imaging and spectroscopic performance. This paper presents the in-flight determination of the Xtend-XMA optical axis and the quantitative evaluation of diffuse stray light characteristics utilizing these calibration datasets. First, by analyzing multi-pointing observations of the galaxy cluster Abell 2029, we developed an asymmetric vignetting model that accounts for the intrinsic structural boundaries of the mirror quadrants, successfully eliminating a severe energy-dependent systematic bias inherent in conventional symmetric models. The newly derived reference optical axis at (DETX, DETY) = (730.1, 727.5) significantly reduces the alignment separation from the in-flight aimpoint from the initial 5300 to less than 9 00. Second, dedicated 600 off-axis observations of the Crab Nebula were utilized to characterize the XMA’s stray light performance under a worst-case azimuthal configuration. The observed stray light in the Xtend wide-field images was successfully decomposed into a soft precollimator-blade scattering component (∼1 keV) and a harder secondary-only single-reflection component (up to 5 keV). The compact Resolve Field of View shows no detectable stray light under the current gate-valve (GV) closed state, which is consistent with the non-X-ray background (NXB). On the other hand, predictive spectral simulations indicate that the secondary-only component will surpass the NXB in the 1–3 keV band once the GV is opened, reaching approximately four times the NXB level near 1.5 keV.