Soft X-ray Imager of the Xtend system on board XRISM

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Abstract The Soft X-ray Imager (SXI) is the X-ray charge-coupled device (CCD) camera for the soft X-ray imaging telescope Xtend installed on the X-ray Imaging and Spectroscopy Mission (XRISM), which was adopted as a recovery mission for the Hitomi X-ray satellite and was successfully launched on 2023 September 7 (JST). In order to maximize the science output of XRISM, we set the requirements for Xtend and found that the CCD set employed in the Hitomi/SXI or similar, i.e., a $2 \times 2$ array of back-illuminated CCDs with a $200\, \mu$m-thick depletion layer, would be practically best among the available choices, when used in combination with the X-ray mirror assembly. We designed the XRISM/SXI, based on the Hitomi/SXI, to have a wide field of view of $38^{\prime } \times 38^{\prime }$ in the 0.4–13 keV energy range. We incorporated several significant improvements from the Hitomi/SXI into the CCD chip design to enhance the optical-light blocking capability and to increase the cosmic-ray tolerance, reducing the degradation of charge-transfer efficiency in orbit. By the time of the launch of XRISM, the imaging and spectroscopic capabilities of the SXI had been extensively studied in on-ground experiments with the full flight-model configuration or equivalent setups and confirmed to meet the requirements. The optical blocking capability, the cooling and temperature control performance, and the transmissivity and quantum efficiency to incident X-rays of the CCDs were also all confirmed to meet the requirements. Thus, we successfully completed the pre-flight development of the SXI for XRISM.

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