内田 裕之

Abstract The elemental abundances of the Fe-peak elements (such as Cr, Mn, Fe, and Ni) and Ti are important for understanding the environment of explosive nuclear burning for the core-collapse supernovae (CCSNe). In particular, the supernova remnant Cassiopeia A (Cas A), which is well known for its asymmetric structure, contains three “Fe-rich blobs,” and the composition of the Fe-peak elements within these structures could be related to the asymmetry of the supernova explosion. We report a highly asymmetric distribution of the Fe-peak elements in Cas A as revealed by XRISM observations. We found that the southeastern Fe-rich region has a significant Mn emission above the 4 σ confidence level, while the northwestern Fe-rich region has no clear signature. In addition to the significant difference in Mn abundance across these regions, our observations show that the Ti/Fe, Mn/Cr, and Ni/Fe ratios vary from region to region. The observed asymmetric distribution of Fe-peak elements could be produced by (1) the mixing of materials from different burning layers of the supernova, (2) the asymmetric distribution of the electron fraction in the progenitor star, and/or (3) the local dependence of the neutrino irradiation in the supernova innermost region. Future spatially resolved spectroscopy of Cas A using X-ray microcalorimeters will enable more detailed measurements of the distribution and composition of these elements, providing a unique tool for testing asymmetric supernova physics.

ABSTRACT The Fe K$\alpha$ fluorescence line ($\sim 6.4$ keV) has been observed during solar and stellar flares. Two emission mechanisms of the Fe K$\alpha$ line, photoionization, and collisional ionization, have been discussed, and the aim of this work is to collect evidences for each mechanism employing a statistical correlation approach between the Fe K$\alpha$ line flux and rough flare properties. Here, we systematically searched the Neutron Star Interior Composition ExploreR (0.2–12 keV) archive data for the Fe K$\alpha$ line of RS Canum Venaticorum-type stars. Among our analysed 255 observation IDs with a total exposure of $\sim 700$ ks, we found 25 data sets (total $\sim 40$ ks) exhibiting the Fe K$\alpha$ emission line at 6.37–6.54 keV with its equivalent width of 44.3–578.4 eV: 18 observations during flares, six observations during unconfirmed possible flare candidates, and one at a quiescent phase. These observations indicate a positive correlation between the Fe K$\alpha$ line intensity ($L_{\rm{K \alpha } }$) and the 7.11–20 keV thermal plasma luminosity ($L_{\mathrm{HXR } }$) with its power-law index of $0.86 \pm 0.46$ (i.e. $L_{\mathrm{K \alpha } } \propto L_{\mathrm{HXR } }^{0.86\pm 0.46}$). This correlation in the range of the thermal plasma luminosity $10^{29-33}$ erg s$^{-1}$ is consistent with the photoionization origin of the line. On the other hand, the equivalent width of the Fe K$\alpha$ line ($\mathrm{EW}_{\mathrm{K \alpha } }$) has a negative correlation with the 7.11–20 keV thermal plasma luminosity with its power-law index of $-0.27 \pm 0.10$ (i.e. $\mathrm{EW}_{\mathrm{K \alpha } } \propto L_{\mathrm{HXR } }^{-0.27\pm 0.10}$). This anticorrelation is consistent with the decline of the fluorescence efficiency with increasing the stellar flare loop height. Furthermore, we found a signature of an absorption line at $6.38^{+0.03}_{-0.04}$ keV during a superflare of $\sigma$ Gem. The equivalent width of the line was $-34.7^{+2.03}_{-1.58}$ eV. We discuss the density of the Fe ions from the equivalent width using the curve of growth analysis.

Abstract Young supernova remnants (SNRs) provide crucial insights into explosive nucleosynthesis products and their velocity distribution soon after the explosion. However, these velocities are influenced by the dynamics of the circumstellar medium (CSM), which in young core-collapse SNRs originates from the progenitor’s late-phase mass loss. Cassiopeia A (Cas A), the youngest known Galactic core-collapse SNR, was studied to analyze the spatial distribution of silicon and sulfur radial velocities using two high-spectral-resolution observations from the XRISM/Resolve imaging spectrometer. Resolve’s capabilities enabled the detailed characterization of Si xiii, Si xiv, S xv, and S xvi lines, whose line shapes can be resolved and modeled using Gaussian radial velocity components. The radial velocities measured generally align with previous CCD-based results, confirming that they were not artifacts caused by blended lines or ionization variations. Modeling line profiles with two-component Gaussians improved fits in some regions, revealing distinct redshifted (back side) and blueshifted (front side) components only in a few specific areas. In most regions, however, both components were either both redshifted (north-west) or both blueshifted (south-east), consistent with the patchy ejecta shell morphology seen in optically emitting fast-moving knots. The individual line components revealed line broadening ranging from $\sigma _v \approx 200$ to $\sigma _v \approx 2000$ ${\rm km\, s^{-1 } }$. Components with $1000 \lesssim \sigma _v \lesssim 2000$ ${\rm km\, s^{-1 } }$ are consistent with previously determined reverse-shock velocities, suggesting non-equilibrated or partially equilibrated ion temperatures. Narrow components with small radial velocities found near Cas A’s projected center likely originate from shocked CSM plasma. But the low radial velocity and small $\sigma _v$ poses a challenge to identify these components with either the front side or back side of the SNR, or a blend of both sides.

Abstract The expansion structure of supernova remnants is important for understanding not only how heavy elements are distributed into space, but also how supernovae explode. The ejecta expansion structure of the young core-collapse supernova remnant Cas A is investigated, with Doppler parameter mapping of the Fe–K complex by the Resolve microcalorimeter onboard the X-ray Imaging and Spectroscopy Mission (XRISM). It is found that the Fe ejecta are blueshifted in the south-east and redshifted in the north-west, indicating an incomplete shell structure, similar to intermediate-mass elements (IMEs) such as Si and S. The Fe has a velocity shift of $\sim$1400 and $\sim$2160 km s$^{-1}$ in the north-west and south-east regions, respectively, with the error range of a few hundred km s$^{-1}$. These values are consistent with those for the IMEs in the north-west region, but larger than those for the IMEs in the south-east region, although the large error region prevented us from concluding which component has significantly higher velocity. The line broadening is larger in the center with values of $\sim$2000–3000 km s$^{-1}$, and smaller near the edges of the remnant. The radial profiles of the Doppler shift and broadening of the IMEs and Fe indicate that the Fe ejecta may expand asymmetrically as IME ejecta, although the large error regions do not allow us to confirm this. Moreover, we see little bulk Doppler broadening of the Fe lines in the north-eastern jet region whereas the IME lines exhibit significant broadening. No such narrow lines are detected in the north-west region. These findings suggest an asymmetric expansion of the ejecta potentially driven by large-scale asymmetries originating from the supernova explosion. This interpretation aligns with the large-scale asymmetries predicted by models of neutrino-driven supernova explosions.

Abstract We present a summary of the in-orbit performance of the soft X-ray imaging telescope Xtend onboard the X-Ray Imaging and Spectroscopy Mission (XRISM), based on in-flight observation data, including first-light celestial objects, calibration sources, and results from the cross-calibration campaign with other currently operating X-ray observatories. XRISM/Xtend has a large field of view of ${38{^{\prime }_{. } }5}$  $\times$  ${38{^{\prime }_{. } }5}$, covering an energy range of 0.4–13 keV, as demonstrated by the first-light observation of the galaxy cluster Abell 2319. It also features an energy resolution of 170–180 eV at 6 keV, which meets the mission requirement and enables us to resolve He-like and H-like Fe K$\alpha$ lines. Throughout the observation during the performance verification phase, we confirm that two issues identified in the Soft X-ray Imager (SXI) onboard the previous Hitomi mission—light leakage and crosstalk events—are addressed and suppressed in the case of Xtend. A joint cross-calibration observation of the bright quasar 3C 273 results in an effective area measured to be $\sim$420 cm$^{2}$ at1.5 keV and $\sim$310 cm$^{2}$ at 6.0 keV, which matches values obtained in ground tests. We also continuously monitor the health of Xtend by analyzing overclocking data, calibration source spectra, and day-Earth observations; the readout noise is stable and low, and contamination is negligible even one year after launch. A low background level compared with other major X-ray instruments onboard satellites, combined with the largest grasp ($\Omega _{\rm eff}\sim 60$ cm$^2$ deg$^2$) of Xtend, will not only support Resolve analysis, but also enable significant scientific results on its own. This includes near-future follow-up observations and transient searches in the context of time-domain and multi-messenger astrophysics.

Abstract XRISM has delivered one of its first light observations on N 132D, the X-ray brightest supernova remnant in the Large Magellanic Cloud. Utilizing 193 ks of high-resolution X-ray spectroscopy data, we conduct a comprehensive search for charge exchange emission. By incorporating a charge exchange model into our spectral analysis, we observe an improvement in the fits of two weak features at 2.41 and 2.63 keV. These features, with a combined significance of 99.6%, are consistent with transitions from highly ionized silicon ions in high Rydberg states, which are unique indicators of charge exchange. Our analysis constrains the charge exchange flux to no more than 4% of the total source flux within the 1.7–3.0 keV band, and places an upper limit on the charge exchange interaction velocity at 450 km s$^{-1}$. This result supports ongoing shock–cloud interactions within N 132D and highlights the unique capabilities of XRISM to probe the complex physical processes at play.

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.

Abstract Mechanisms of particle heating are crucial to understanding the shock physics in supernova remnants (SNRs). However, there has been little information on time variabilities of thermalized particles so far. Here, we present a discovery of a gradually brightening thermal X-ray emission found in the Chandra data of Tycho’s SNR obtained during 2000–2015. The emission exhibits a knot-like feature (Knot1) with a diameter of ≃0.04 pc located in the northwestern limb, where we also find localized Hα filaments in an optical image taken with the Hubble Space Telescope in 2008. The model with the solar abundance reproduces the spectra of Knot1, suggesting that Knot1 originates from the interstellar medium; this is the first detection of thermal X-ray emission from swept-up gas found in Tycho’s SNR. Our spectral analysis indicates that the electron temperature of Knot1 has increased from ∼0.30 to ∼0.69 keV within the period between 2000 and 2015. These results lead us to ascribe the time-variable emission to a small dense clump recently heated by the forward shock at the location of Knot1. The electron-to-proton temperature ratio immediately downstream of the shock (β ₀ ≡ T _e /T _p ) is constrained to be m _e /m _p ≤ β ₀ ≤ 0.15 to reproduce the data, indicating the collisionless electron heating with efficiency is consistent with previous Hα observations of Tycho and other SNRs with high shock velocities.