Hiroyuki Uchida

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.