We present the design and performance of the XRISM Soft X-Ray Spectrometer Resolve, successfully launched on a JAXA H-IIA rocket September 7, 2023. The instrument uses a 36-pixel array of microcalorimeters at the focus of a grazing-incidence x-ray mirror. The instrument has achieved an energy resolution of 4.5 eV (FWHM) at 6. The overall cooling chain was designed to provide a lifetime of at least 3 years in orbit and operate without liquid helium to provide redundancy and the longest operational lifetime for the instrument. Early indications that the cryogen lifetime will exceed 4 years. X-rays are focused onto the array with a high-throughput grazing incidence X-ray Mirror Assembly with over 200 nested two-stage X-ray reflectors. A series of onboard X-ray calibrations sources allow simultaneous energy scale calibration lines simultaneously while observing celestial sources. The inflight performance of Resolve will be described along with a summary of the scientific capabilities.
The initial on-orbit checkout of the soft X-ray spectroscopic system on board the XRISM satellite is summarized. The satellite was launched on September 6, 2023 (UT) and has been undergoing initial checkout since then. Immediately after the launch, the cryocoolers were turned on and their operation was established. The first cycle of the adiabatic demagnetization refrigerator was performed on Oct. 9th, 2023, to transition the sensor to steady-state operational temperature conditions. Subsequently, the filter wheel, which supports energy calibration, was started up. The energy scale is highly sensitive to the temperature environment around the sensor and its analog electronics. The gain correction was established by referring to the calibration X-ray line. For an optimization of the cooler frequencies, we took data including the noise spectra by scanning the cooler frequencies, and selected a good frequency pair in the on-orbit environment. At the last stage of the checkout, the gate valve, which protects the inside of the Dewar from outside air pressure at launch, was attempted to be opened to bring the system to a state where it is ready for regular operations but was failed.
The in-orbit performance verification and calibration of the X-ray Mirror Assembly (XMA) on the XRISM satellite are ongoing. The optical performance of the XMA is being measured with bright point-like or bright small enough sources in orbit. This paper reports the preliminary results for the XMA of Resolve (Resolve- XMA). The in-orbit on-axis and off-axis (1.8′–9′ away from the aimpoint) Point Spread Function (PSF) of the Resolve-XMA were measured using 3C 273 and Cyg X-2, respectively. No significant changes were found in the PSF compared to on-ground results, indicating that the Resolve-XMA is functioning as expected (HPD ∼ 1.3′). For the Effective Area (EA) calibration, XRISM observed 3C 273 alongside other satellites. The Resolve spectrum of 3C 273 is well reproduced by an absorbed power law. While the best-fit photon index is consistent with that measured by NuSTAR, the flux is 5% higher than the NuSTAR measurement. In-orbit stray light observation was conducted with Crab at 60′ off from the aimpoint, and a signature of stray light on Resolve was detected. The observation of Resolve optical search has not yet been performed, and GX 3+1 has been proposed as the target.
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.
The XRISM (X-Ray Imaging and Spectroscopy Mission) satellite was successfully launched and put into low- Earth orbit on September 6, 2023 (UT). The Resolve instrument onboard XRISM hosts an x-ray microcalorimeter detector, which was designed to achieve a high-resolution (≤7 eV FWHM at 6 keV), high-throughput, and nondispersive spectroscopy over a wide energy range. It also excels in a low background with a requirement of < 2 × 10−3 s−1 keV−1 (0.3–12.0 keV), which is equivalent to only one background event per spectral bin per 100 ks exposure. Event screening to discriminate x-ray events from background is a key to meeting the requirement. We present the result of the Resolve event screening using the data sets on the ground and in the orbit based on the heritage of the preceding x-ray microcalorimeter missions, in particular, the Soft X-ray Spectrometer (SXS) onboard ASTRO-H. We optimize and evaluate 19 screening items of three types based on (1) the event pulse shape, (2) relative arrival times among multiple events, and (3) good time intervals. We show that the initial screening, which is applied for science data products in the performance verification phase, reduces the background rate to 1.8×10−3 s−1 keV−1 meeting the requirement. We further evaluate the additional screening utilizing the correlation among some pulse shape properties of x-ray events and show that it further reduces the background rate particularly in the <2 keV band. Over 0.3–12 keV, the background rate becomes 1.0 × 10−3 s−1 keV−1.
The Resolve instrument aboard the X-ray Imaging and Spectroscopy Mission (XRISM) is a 36-pixel microcalorimeter spectrometer that provides non-dispersive spectroscopy with 5 eV spectral resolution in the soft x-ray waveband. Resolve has a requirement to provide absolute energy-scale calibration of ± 2 eV from 0.3–12 keV. In this manuscript we describe our ground calibration strategy and results of a subset of the ground calibration campaigns, including a discussion of improvements in the energy scale ground calibration compared to Hitomi’s. These improvements include calibration of the low-energy band below 4 keV with the instrument in the flight dewar and the dewar aperture door open, which was not performed for Hitomi, and thorough measurements over an extended high-energy waveband to 22 keV. We also developed an improved technique for gain calibration of ‘mid-res’ secondary events, which have suppressed gain due to proximity to a preceding x-ray event (18-70 ms) on the same pixel. We provide an assessment of how well these pre-launch gain scales correct on-orbit data and discuss approaches for updating the gain curves. Energy-scale calibration approaches for future space-based instruments, including the X-ray Integral Field Unit (X-IFU) on Athena and the microcalorimeter spectrometer proposed for the Line Emission Mapper (LEM), have heritage in the calibration of XRISM. We briefly comment on lessons learned from Resolve calibration that are relevant for these future instruments.
The Resolve instrument was launched on-board the XRISM observatory in early September 2023. The Resolve spectrometer is based on a high-sensitivity X-ray calorimeter detector system that has been successfully deployed in many ground and sub-orbital spectrometers. However, the Resolve instrument will be the first long-term implementation in space. The instrument will provide essential diagnostics for nearly every class of X-ray emitting objects, from the atmosphere of Jupiter to the outskirts of galaxy clusters, without degradation for spatially extended objects. The Resolve detector system consists of a 36-pixel microcalorimeter array operated at a heat-sink temperature of 50mK. In pre-flight testing, the detector system demonstrated a resolving power of better than 1300 at 6 keV with a simultaneous bandpass from below 0.3 keV to above 12 keV and a timing precision better than 100 μs. An anti-coincidence detector placed directly behind the microcalorimeter array effectively suppresses background. The detector energy-resolution budget included terms for interference from the Resolve cooling system and the spacecraft. Additional terms for energy-scale stability, on-orbit effects, and use of mid-grade events were also included, predicting an end-of-life, on-orbit performance for high and mid-grade events that meets the requirement of 7 eV FWHM at 6 keV. Here we discuss the actual on-orbit performance of the Resolve detector system and compare this to performance in pre-flight testing, on-orbit predictions, and the almost identical Hitomi/SXS instrument. We will also discuss the on-orbit gain stability, any additional on-orbit interference, and measurements of the on-orbit background.
The Resolve soft X-ray spectrometer is the high spectral resolution microcalorimeter spectrometer for the XRISM mission. In the beam of Resolve there is a filter wheel containing X-ray filters. Also in the beam is an active calibration source (the modulated X-ray source (MXS) which can provide pulsed X-rays to facilitate gain calibration.
The filter wheel consists of 6 filter positions. Two open positions, one 55Fe source to aid in early mission spectrometer characterisation and three transmission filters: a neutral density filter, an optical blocking filter and a beryllium filter.
The X-ray intensity, pulse period and pulse separation of the MXS are highly configurable. Furthermore, the switch–on time is synchronized with the space–craft’s internal clock to give accurate start and end times of the pulses.
One of the issues raised during ground testing was the susceptibility of an MXS at high voltage to ambient light. Although measures were taken to mitigate the light leak, the efficacy of those measures must be verified in–orbit. Along with an overview of issues raised during ground testing, this article will discuss the calibration source and the filter performance in–flight and compare with the transmission curves present in the Resolve calibration database.
The X-Ray Imaging and Spectroscopy Mission (XRISM) project at JAXA officially started in 2018. Following the development of onboard components, the proto-flight test was conducted from 2021 to 2023 at JAXA Tsukuba Space Center. The spacecraft was launched from JAXA Tanegashima Space Center on September 7, 2023 (JST), and onboard components, including the science instruments, were activated during the in-orbit commissioning phase. Following the previous report in 2020, we report the spacecraft ground tests, the launch operation, in-orbit operations, and the status and plan of initial and subsequent guest observations.
Resolve is the instrument that utilizes an X-ray micro-calorimeter array onboard the XRISM (X-Ray Imaging and Spectroscopy Mission), which was launched on September 6 (UT), 2023. It fully met the spectral performance requirement (7 eV at 6 keV) both on the ground and in orbit and was confirmed to have the same performance as the SXS onboard the ASTRO-H (Hitomi) satellite. The detectors are operated at a low temperature of 50 mK to achieve the required energy resolution with the cooling system to satisfy the lifetime requirement of over 3 years. The cooling system is equipped with a 3-stage ADR and superfluid liquid He (LHe) as the heat sink for the ADR. The Joule-Thomson cooler unit and 2-stage Stirling cooler units are adopted to reduce heat load to the LHe. In the pre-launch operations, we carried out the low-temperature LHe top-off operation. The resultant amount of liquid He was over 35 L at the launch, which is sufficient to meet the lifetime requirement. During the post-launch operation, the LHe vent valve was opened five minutes after launch during the rocket acceleration, and the cryocoolers started in several revolutions as planned which established stable cooling of the dewar.
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