We have been developing an X-ray imaging system, multi-image X-ray interferometer module (MIXIM), to achieve a high angular resolution with a compact system size. MIXIM is comprised of a mask with equally spaced apertures and an X-ray detector. The aperture size and the mask-detector distance determine the system’s angular resolution. Although a smaller aperture gives a better resolution, the degree of improvement is limited by a diffraction effect. MIXIM circumvents this problem by utilizing the Talbot effect. Our experiment with the previous model equipped with a multi-pinhole mask obtained an angular resolution of 0.5″ with a mask-detector distance of 92 cm. A major downside of the multi-pinhole mask is, however, that it has a very low opening fraction, which results in a very low effective area. Here, we newly adopt a multiple coded aperture (MCA) mask, an array of coded aperture patterns. Our proof-of-concept experiment demonstrates that the Talbot effect works even for the MCA mask with a high opening fraction of ∼50% at 12.4 keV. Consequently, the new MIXIM realizes ∼25 times as large an effective area as that of the previous model while maintaining a high angular resolution of 0.2″ and a compact size of ∼1.5 m.
We have developed a small liquid argon time projection chamber (LArTPC) called NanoGRAMS, whose fiducial volume is 5 × 5 × 10 cm3. A LArTPC has advantages of dense and large volumes sensitive to MeV gamma rays compared with multi-layer semiconductor detectors or gaseous ones. The aim of our work is proof-ofconcept studies for one of the important goals of Gamma-Ray and AntiMatter Survey (GRAMS). The imaging performance of Compton cameras is mainly determined by energy and position resolutions of the detector. The NanoGRAMS measures scintillation light and ionized electrons generated by interactions of gamma rays and argon atoms, which are reconstructed to information on the initial energies and momenta of incoming photons. The NanoGRAMS is equipped with photon detection boards loading a large-area array consisting of 4×4 single SiPMs, which totally cover 2.56×2.56 cm2, and low-noise electron readout boards. The SiPM boards operate at a liquid argon (87 K) temperature and exhibit a fast response time below 100 ns allowing pulse shape discrimination to reject atmospheric neutron background. The electron readout system comprises an anode electrode segmented into 16×16 pixels with a 3.2mm pitch. Charge signals from all pixels are processed by four 64-channel low-noise readout ASICs originally developed for semiconductor detectors. We confirmed that this TPC has the capability of detecting scintillation light and electrons generated by α-rays in gaseous argon and gamma rays in liquid argon.
GRAMS (Gamma-Ray and AntiMatter Survey) is a next-generation proposed balloon-borne/satellite-based mission aimed at high sensitivity MeV gamma-ray astrophysical observations and background-free indirect dark matter search via hadronic antiparticles. The main detector of GRAMS is a meter-scale liquid argon time projection chamber (LArTPC). The adoption of liquid argon as detector material allows us to produce an unprecedentedly large effective area instrument both for cosmic MeV gamma rays of 0.5-20 MeV and antiparticles of dark matter origin. This large effective area, which will exceed 1000 cm2, is necessary for measuring faint gamma-ray signals of nuclear line emissions from energetic phenomena such as supernovae as well as for observing short-duration transient objects including gamma-ray bursts with high photon statistics. In this talk, we present the mission concept and design, the current proof-of-concept studies using prototype LArTPCs, and an engineering balloon flight conducted in 2023.
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.
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 Soft X-ray Imager (SXI) is an X-ray CCD camera of the Xtend system onboard the X-Ray Imaging and Spectroscopy Mission (XRISM), which was successfully launched on September 7, 2023 (JST). During ground cooling tests of the CCDs in 2020/2021, using the flight-model detector housing, electronic boards, and a mechanical cooler, we encountered an unexpected issue. Anomalous charges appeared outside the imaging area of the CCDs and intruded into the imaging area, causing pulse heights to stick to the maximum value over a wide region. Although this issue has not occurred in subsequent tests or in orbit so far, it could seriously affect the imaging and spectroscopic performance of the SXI if it were to happen in the future. Through experiments with non-flight-model detector components, we successfully reproduced the issue and identified that the anomalous charges intrude via the potential structure created by the charge injection electrode at the top of the imaging area. To prevent anomalous charge intrusion and maintain spectroscopic performance that satisfies the requirements, even if this issue occurs in orbit, we developed a new CCD driving technique. This technique is different from the normal operation in terms of potential structure and its changes during imaging and charge injection. In this paper, we report an overview of the anomalous charge issue, the related potential structures, the development of the new CCD driving technique to prevent the issue, the imaging and spectroscopic performance of the new technique, and the results of investigation experiments to identify the source of the anomalous charges.
XRISM (X-ray Imaging and Spectroscopy Mission) is an X-ray astronomy satellite developed in collaboration with JAXA, NASA and ESA. It successfully launched on Sept. 7, 2023. Two complementary X-ray telescopes, Resolve and Xtend are on-board XRISM. Resolve uses the pixelized X-ray micro calorimeter developed by NASA/GSFC and has very high energy resolution of 5 eV. On the other hand, Xtend uses an X-ray CCD camera as its focal plane detector which has high spatial resolution and a wide field of view. We evaluated the performance of the X-ray Mirror Assembly (XMA) for Xtend using data observed during the commissioning and PV phases of XRISM. To verify the imaging performance, the Point Spread Functions (PSF) generated from the observations of NGC 4151 and PDS 456 were compared with the ground-calibration results. The results show that the imaging performance of Xtend-XMA is not significantly different from that of the ground calibration, and that it meet the requirement. The effective area was verified by comparing the results of simultaneous observations of 3C 273 by XRISM and four X-ray astronomy satellites (Chandra, XMM-Newton, NuSTAR, and Swift). The results of the fitting of the X-ray spectrum of Xtend show no significant difference from the results of other satellites, suggesting the effective area used for fitting is correct. The on-axis position on the detector was estimated from the intensity of the Abell 2029 observations at four off-axis angles. The on-axis is about 40 arcsec away from the aim point, and the decrease in effective area at the aim point is less than 1%. Stray light observations of the Crab Nebula at 60 arcmin off-axis were obtained at two different satellite roll angles. The stray light intensity obtained at each roll angle was significantly different, verifying the dependence of the stray light on the roll angle.
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.
We have developed a novel x-ray interferometer, multi-image x-ray interferometer module (MIXIM), comprised of a fine aperture mask and an x-ray detector. The angular resolution of this system can be improved with an increase of the distance between two components or a decrease of the aperture size. Although MIXIM has already achieved an angular resolution of less than 0.1” by applying the Talbot effect with a periodic multi-pinhole mask, there remains the issue that its low opening fraction of 1.3% decreases the effective area of the imaging system. Therefore, we newly introduced periodic coded aperture masks which have opening fractions of about 50% instead of the multi-pinhole mask. Conducting an experiment with a 12.4 keV parallel x-ray beam, we successfully demonstrated that the periodic coded aperture could form the self-image, and obtained the x-ray source profile with sub-arcsecond angular resolution by deciphering the coded pattern. The effective area increases about 25 times compared with the multi-pinhole mask by the introduction of the periodic coded aperture masks, which indicates that this novel method can be effective for addressing the problem.
Xtend is a soft x-ray imaging telescope developed for the x-ray imaging and spectroscopy mission (XRISM). XRISM is scheduled to be launched in the Japanese fiscal year 2022. Xtend consists of 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. The SXI uses the P-channel, back-illuminated type CCD with an imaging area size of 31mm on a side. The four CCD chips are arranged in a 2×2 grid and can be cooled down to −120 °C with a single-stage Stirling cooler. The XMA nests thin aluminum foils coated with gold in a confocal way with an outer diameter of 45 cm. A pre-collimator is installed in front of the x-ray mirror for the reduction of the stray light. Combining the SXI and XMA with a focal length of 5.6m, a field of view of 38′ × 38′ over the energy range from 0.4 to 13 keV is realized. We have completed the fabrication of the flight model of both SXI and XMA. The performance verification has been successfully conducted in a series of sub-system level tests. We also carried out on-ground calibration measurements and the data analysis is ongoing.
We intoduce our novel method of super high resolution astronomical X-ray imaging, Multi Image X-ray Interferometer Method, Modules, Missions (MIXIM). In series of experiments on the ground we not only verified the concept of MIXIM but also realized 2D imaging with angular resolution better than 0. ′′1. Employment of small pixel size CMOS sensor was the key to this achievement. Scalability is also an important feature of MIXIM., and various mission format is available. We show some examples from a very small satellite for sub arcsecond resolution to a formation flight with a millions km separation to gain µas resolution. MIXIM is different from X-ray mirrors in various points, for example, it does not have a collecting power. Considering the limitations and advantages of MIXIM, we should choose bright apparently point-like sources as targets. Nearby AGNs are primary ones, and the MIXIM scope just corresponds to spatial scales which have not yet resolved in X-rays.
CFRP is a composite material composed of carbon fiber and resin. CFRP is commonly applied to the aerospace industry which requires lightweight and intensity. Thanks to superior formability of CFRP, we can form shape of Wolter-1 optics, which consists of paraboloid and hyperboloid, to a monolithic substrate. Since the surface roughness of a CFRP substrate is a few µm, we have to make the smooth surface for reflecting X-rays on the CFRP substrate. We have developed a new method of shaping the reflective surface instead of the replica method used in lightweight X-ray mirrors such as Astro-H. In the new method, the reflective surface is formed by pasting thin sheet-glasses with 100 µm thick onto the CFRP substrate. The thin sheet-glass has a surface roughness about 0.4 nm as measured by Zygo. We fabricated a CFRP mirror pasting thin sheet-glasses, and then coated tungsten on the mirror in June 2020. The figure error (s) of the CFRP mirror was achieved to be about 1-2 μm by stacking the CFRP mirror on the housing module. X-ray imaging quality of the CFRP mirror was measured at Spring-8 in July 2020. The half-power diameter of the CFRP mirror was estimated to be about 150 arcsec, which was nearly equal to the prediction from a distribution of the slope error deduced from the surface profile. We describe a future plan to improve the image quality of the CFRP mirror.
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