The X-ray Imaging Spectroscopy Mission (XRISM) was launched on 2023-09-06 (UT). The onboard Xtend camera is now performing spectral imaging in the 0.4–13 keV band. Utilizing the wide field of view of Xtend of 38′ ×38′, we have been conducting searches for transient sources once a day, under the name of “XRISM/Xtend Transient Search (XTS)”. We report an Astronomer’s Telegram (ATel) within ∼1 day after downlinking the data from the satellite. Since the beginning of March 2024, when the XTS began full-scale operations, through early July, we issued nine ATel reports. The sensitivity limit of XTS is about 10−14 erg s−1cm−2 for a one-day observation, i.e. for an exposure time of ∼40 ks with the typical XRISM observing efficiency of about 50%. This sensitivity limit is orders of magnitude fainter than those obtained with all-sky survey missions such as MAXI and Einstein probe. It is comparable to those of Swift/XRT and eROSITA/SRG. XRISM observations are 4 days long on average, allowing for searches of sources that vary on time scales of more than one day, and down to the 10−14 erg s−1cm−2 flux range. Consequently, XRISM now plays a key role in time-domain astronomy.
XRISM (X-Ray Imaging and Spectroscopy Mission) is an astronomical satellite with the capability of highresolution spectroscopy with the X-ray microcalorimeter, Resolve, and wide field-of-view imaging with the CCD camera, Xtend. The Xtend consists of the mirror assembly (XMA: X-ray Mirror Assembly) and detector (SXI: Soft X-ray Imager). The components of SXI include CCDs, analog and digital electronics, and a mechanical cooler. After the successful launch on September 6th, 2023 (UT) and subsequent critical operations, the mission instruments were turned on and set up. The CCDs have been kept at the designed operating temperature of −110°C after the electronics and cooling system were successfully set up. During the initial operation phase, which continued for more than a month after the critical operations, we verified the observation procedure, stability of the cooling system, all the observation options with different imaging areas and/or timing resolutions, and operations for protection against South Atlantic Anomaly. We optimized the operation procedure and observation parameters including the cooler settings, imaging areas for the specific modes with higher timing resolutions, and event selection algorithm. We summarize our policy and procedure of the initial operations for SXI. We also report on a couple of issues we faced during the initial operations and lessons learned from them.
Xtend is one of the two telescopes onboard the X-ray imaging and spectroscopy mission (XRISM), which was launched on September 7th, 2023. Xtend comprises the Soft X-ray Imager (SXI), an X-ray CCD camera, and the X-ray Mirror Assembly (XMA), a thin-foil-nested conically approximated Wolter-I optics. A large field of view of 38′ × 38′ over the energy range from 0.4 to 13 keV is realized by the combination of the SXI and XMA with a focal length of 5.6 m. The SXI employs four P-channel, back-illuminated type CCDs with a thick depletion layer of 200 μm. The four CCD chips are arranged in a 2×2 grid and cooled down to −110°C with a single-stage Stirling cooler. Before the launch of XRISM, we conducted a month-long spacecraft thermal vacuum test. The performance verification of the SXI was successfully carried out in a course of multiple thermal cycles of the spacecraft. About a month after the launch of XRISM, the SXI was carefully activated and the soundness of its functionality was checked by a step-by-step process. Commissioning observations followed the initial operation. We here present pre- and post-launch results verifying the Xtend performance. All the in-orbit performances are consistent with those measured on ground and satisfy the mission requirement. Extensive calibration studies are ongoing.
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