arXiv subjects

Yuken Ohshiro

Publications and source records attributed to Yuken Ohshiro.

10 recordsLinked to original sources

arXiv2026

Progenitor and Explosion Mechanism of 3C 397 Indicated from XRISM High-resolution Spectroscopy of Fe-group Elements

We present spatially resolved X-ray spectroscopy of the Type Ia supernova remnant 3C 397, one of the most promising candidates to have originated from a white dwarf with a mass close to the Chandrasekhar limit. The remnant was observed during the performance verification phase of the X-ray Imaging and Spectroscopy Mission (XRISM), with the field of view of the Resolve calorimeter array positioned on the eastern half of the remnant. We divide the Resolve field of view into southeastern and northeastern regions and extract spectra from each region for both Resolve and Xtend. The Resolve spectra are characterized by narrow K-shell emission lines from intermediate-mass elements (IMEs) such as Si, S, Ar, and Ca, and by broader K-shell emission lines from iron-group elements (IGEs) such as Cr, Mn, Fe, and Ni. K-shell emission lines from Ti and Cr are also detected, and are found to be locally enhanced in the southeastern region, as reported in previous observations. We model the Resolve and Xtend spectra simultaneously by a non-equilibrium ionization (NEI) plasma model with multiple temperatures and ionization states. The observed mass ratios of Ti/Fe and Cr/Fe in the southeastern region can only be explained by nucleosynthesis in a neutron-rich environment produced by electron-capture reactions in the innermost layers of the exploding white dwarf. By comparing the observed mass ratios with those predicted by nucleosynthesis models, we constrain the central density to be >= 4.0 x 10^9 g cm^{-3} for the deflagration-to-detonation transition model and >= 6.0 x 10^9 g cm^{-3} for the pure turbulent deflagration model. On the other hand, the observed Ni/Fe mass ratio is globally enhanced across the field of view, suggesting an additional neutronization mechanism beyond electron-capture reactions, such as a higher progenitor metallicity.

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arXiv2026

Tracing the Orbital Motion of the Accreting White Dwarf in EX~Hydrae with XRISM/Resolve

Measuring the masses of accreting white dwarfs (WDs) is crucial for understanding their evolution and the physics of accretion. High-resolution X-ray spectroscopy can trace the WD motion through Doppler shifts of emission lines formed close to the WD. We report an 83~ks XRISM/Resolve observation of the intermediate polar EX~Hydrae and measure the orbital modulation of individual Fe K-shell line centroids. The Fe~{\sc xxv} K$α$ components show coherent orbital modulation, yielding $K_1 = 58.1 \pm 8.5\ \mathrm{km\ s^{-1}}$. This is the first detection of orbital modulation in individual Fe K-shell lines from an accreting WD, made possible by the high spectral resolution of Resolve and its frequent in-orbit gain calibration. The measured $K_1$ is consistent with optical/UV $K_1$ measurements, providing a cross-check that these distinct tracers follow the WD orbital motion. Combining this X-ray measurement with literature orbital parameters, we derive a WD mass of $M_1 = 0.79 \pm 0.04\ M_\odot$. These results demonstrate that high-resolution X-ray spectroscopy can use individual Fe K-shell line centroids to trace WD orbital motion in accreting WDs.

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arXiv2026

Mapping plasma properties of Cassiopeia A with XRISM/Resolve: a Bayesian analysis via UltraSPEX

Mapping the physical conditions of the shocked plasma of young supernova remnants (SNR) is crucial for understanding their explosion mechanisms, ejecta structure, and large-scale asymmetries. Using $>350$ ks of XRISM/Resolve high spectral resolution observations of Cassiopeia A (Cas A), the youngest known Galactic core-collapse SNR, we present the first microcalorimeter-based plasma parameter maps of any SNR. We tessellate Cas A into $1'\times1'$ regions and fit the broadband spectra as thermal emission from two pure-metal ejecta components -- corresponding to intermediate-mass elements (IMEs) and iron-group elements (IGEs) -- plus nonthermal synchrotron radiation. For robust inference, we introduce UltraSPEX, a Bayesian framework that couples the SPEX plasma code with the UltraNest nested-sampling algorithm, yielding full posterior distributions and exploration of parameter degeneracies. Key findings include enhanced Ar/Si and Ca/Si abundance ratios near the base of the Si-rich jets, and a high Ni/Fe mass ratio ($0.08\pm0.015$) in the base of NE jet. IGEs ejecta exhibit systematically higher Doppler velocities and broadenings than IMEs ejecta in most regions, with maximum differences of $\sim800$ km/s and $\sim1200$ km/s, respectively; Ca shows distinct (faster) kinematics from other IMEs in several SE regions. The ionization timescale and electron temperature show a robust anti-correlation, particularly for IGEs. This relation and measured parameter values could be explained by semi-analytical models with significant ejecta clumping (overdensities of $\sim10$ for IGEs and up to $\sim100$ for IMEs) and reduced historical reverse-shock velocities. Nonthermal emission accounts for a substantial fraction, with at least 47% of the 4--6 keV continuum and dominates in the western regions, where the spectrum hardens.

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arXiv2025

Spatially Resolved Plasma Diagnostics of the Supernova Remnant DEM L71 using the Reflection Grating Spectrometer

We present a spatially resolved high-resolution X-ray spectroscopy of the supernova remnant DEM L71 using the Reflection Grating Spectrometer (RGS) aboard XMM-Newton. Because of the large dispersion angle of the RGS, we are able to resolve individual emission lines and examine their spatial distributions within this moderately extended remnant. We derive line fluxes across different regions of DEM L71 and perform quantitative plasma diagnostics. Our analysis reveals that some regions have high forbidden-to-resonance ratios of O\emissiontype{VII} He$α$ lines, suggesting a non-negligible contribution from additional physical processes, such as charge exchange and/or resonance scattering. Our results demonstrate that the RGS has potential to serve as an outstanding X-ray imaging spectrometer for moderately diffuse objects.

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arXiv2025

Evidence for Charge Exchange Emission in Supernova Remnant N132D from XRISM/Resolve Observations

XRISM has delivered one of its first light observations on N132D, 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 keV 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. This result supports ongoing shock-cloud interactions within N132D and highlights the unique capabilities of XRISM to probe the complex physical processes at play.

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arXiv2025

Dynamics of the intermediate-mass-element ejecta in the Supernova Remnant Cassiopeia A studied with XRISM

Supernova remnants (SNRs) provide crucial information of yet poorly understood mechanism of supernova explosion. Here we present XRISM high-resolution spectroscopy of the intermediate-mass-element (IME) ejecta in the SNR Cas A to determine their velocity distribution and thermal properties. The XRISM/Resolve spectrum in the 1.75-2.95 keV band extracted from each $1' \times 1'$ region in the southeast and northwest rims is fitted with a model consisting of two-component plasmas in non-equilibrium ionization with different radial velocities and ionization timescales. It is found that the more highly ionized component has a larger radial velocity, suggesting that this component is distributed in the outer layer and thus has been heated by the SNR reverse shock earlier. We also perform proper motion measurement of the highly ionized component (represented by the Si XIV Ly$α$ emission), using archival Chandra data, to reconstruct the three-dimensional velocity of the outermost IME ejecta. The pre-shock (free expansion) velocity of these ejecta is estimated to range from 2400 to 7100 km s$^{-1}$, based on the thermal properties and the bulk velocity of the shocked ejecta. These velocities are consistent with theoretical predictions for a Type IIb supernova, in which the progenitor's hydrogen envelope is largely stripped before the explosion. Furthermore, we find a substantial asymmetry in the distribution of the free expansion velocities, with the highest value toward the direction opposite to the proper motion of the neutron star (NS). This indicates the physical association between the asymmetric supernova explosion and NS kick.

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arXiv2025

Measurement of the Forward Shock Velocities of the Supernova Remnant N132D Based on the Thermal X-ray Emission

Measuring shock velocities is crucial for understanding the energy transfer processes at the shock fronts of supernova remnants (SNRs), including acceleration of cosmic rays. Here we present shock velocity measurements on the SNR N132D, based on the thermal properties of the shock-heated interstellar medium. We apply a self-consistent model developed in our previous work to X-ray data from deep Chandra observations with an effective exposure of $\sim$ 900 ks. In our model, both temperature and ionization relaxation processes in post-shock plasmas are simultaneously calculated, so that we can trace back to the initial condition of the shock-heated plasma to constrain the shock velocity. We reveal that the shock velocity ranges from 800 to 1500 $\rm{km~s^{-1}}$ with moderate azimuthal dependence. Although our measurement is consistent with the velocity determined by independent proper motion measurements in the south rim regions, a large discrepancy between the two measurements (up to a factor of 4) is found in the north rim regions. This implies that a substantial amount of the kinetic energy has been transferred to the nonthermal component through highly efficient particle acceleration. Our results are qualitatively consistent with the $γ$-ray observations of this SNR.

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arXiv2024

A self-consistent model of shock-heated plasma in non-equilibrium states for direct parameter constraints from X-ray observations

X-ray observations of shock-heated plasmas, such as those found in supernova remnants, often exhibit features of temperature and ionization non-equilibrium. For accurate interpretation of these observations, proper calculations of the equilibration processes are essential. Here, we present a self-consistent model of thermal X-ray emission from shock-heated plasmas that accounts for both temperature and ionization non-equilibrium conditions. For a given pair of shock velocity and initial electron-to-ion temperature ratio, the temporal evolution of the temperature and ionization state of each element was calculated by simultaneously solving the relaxation processes of temperature and ionization. The resulting thermal X-ray spectrum was synthesized by combining our model with the AtomDB spectral code. Comparison between our model and the \texttt{nei} model, a constant-temperature non-equilibrium ionization model available in the XSPEC software package, reveals a 30\% underestimation of the ionization timescale in the \texttt{nei} model. We implemented our model in XSPEC to directly constrain the shock wave properties, such as the shock velocity and collisionless electron heating efficiency, from the thermal X-ray emission from postshock plasmas. We applied this model to archival Chandra data of the supernova remnant N132D, providing a constraint on the shock velocity of $\sim 800~\mathrm{km\,s^{-1}}$, in agreement with previous optical studies.

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arXiv2024

Hydrodynamical simulations favor a pure deflagration origin of the near-Chandrasekhar mass supernova remnant 3C 397

Suzaku X-ray observations of the Type Ia supernova remnant (SNR) 3C 397 discovered exceptionally high mass ratios of Mn/Fe, Ni/Fe, and Cr/Fe, consistent with a near $M_{\rm Ch}$ progenitor white dwarf (WD). The Suzaku observations have established 3C 397 as our best candidate for a near-$M_{\rm Ch}$ SNR Ia, and opened the way to address additional outstanding questions about the origin and explosion mechanism of these transients. In particular, subsequent XMM-Newton observations revealed an unusually clumpy distribution of iron group elemental (IGE) abundances within the ejecta of 3C 397. In this paper, we undertake a suite of two dimensional hydrodynamical models, varying both the explosion mechanism -- either deflagration-to-detonation (DDT), or pure deflagration -- WD progenitors, and WD progenitor metallicity, and analyze their detailed nucleosynthetic abundances and associated clumping. We find that pure deflagrations naturally give rise to clumpy distributions of neutronized species concentrated towards the outer limb of the remnant, and confirm DDTs have smoothly structured ejecta with a central concentration of neutronization. Our findings indicate that 3C 397 was most likely a pure deflagration of a high central density WD. We discuss a range of implications of these findings for the broader SN Ia progenitor problem.

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arXiv2021

Discovery of the Highly-Neutronized Ejecta Clump with Enhanced Abundances of Titanium and Chromium in the Type Ia Supernova Remnant 3C 397

The supernova remnant (SNR) 3C 397 is thought to originate from a Type Ia supernova (SN Ia) explosion of a near-Chandrasekhar-mass ($M_{\rm Ch}$) progenitor, based on the enhanced abundances of Mn and Ni revealed by previous X-ray study with Suzaku. Here we report follow-up XMM-Newton observations of this SNR, conducted with the aim of investigating the detailed spatial distribution of the Fe-peak elements. We have discovered an ejecta clump with extremely high abundances of Ti and Cr, in addition to Mn, Fe, and Ni, in the southern part of the SNR. The Fe mass of this ejecta clump is estimated to be $\sim$ 0.06 $M_{\odot}$, under the assumption of a typical Fe yield for SNe Ia (i.e., $\sim$ 0.8 $M_{\odot}$). The observed mass ratios among the Fe-peak elements and Ti require substantial neutronization that is achieved only in the innermost regions of a near-$M_{\rm Ch}$ SN Ia with a central density of $ρ_c \sim 5 \times 10^9$ g cm$^{-3}$, significantly higher than typically assumed for standard near-$M_{\rm Ch}$ SNe Ia ($ρ_c \sim 2 \times 10^9$ g cm$^{-3}$). The overproduction of the neutron-rich isotopes (e.g., $^{50}$Ti and $^{54}$Cr) is significant in such high-$ρ_c$ SNe Ia, with respect to the solar composition. Therefore, if 3C 397 is a typical high-$ρ_c$ near-$M_{\rm Ch}$ SN Ia remnant, the solar abundances of these isotopes could be reproduced by the mixture of the high- and low-$ρ_c$ near-$M_{\rm Ch}$ and sub-$M_{\rm Ch}$ Type Ia events, with $\lesssim$ 20 % being high-$ρ_c$ near-$M_{\rm Ch}$.

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