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

TL;DR

Using >350 ks of XRISM/Resolve data, this study presents the first microcalorimeter-based plasma maps of Cas A by fitting 1′×1′ regions with two pure-metal NEI pshock components (IME and IGE) plus a nonthermal continuum, all within a Bayesian UltraSPEX framework. UltraSPEX couples SPEX to UltraNest to yield full posterior distributions, capturing degeneracies and multiple minima across 17 free parameters. The results reveal enhanced Ar/Si and Ca/Si near Si-rich jets, a high Ni/Fe ratio at the NE jet base, and systematic differences in Doppler velocities and broadening between IGEs and IMEs, alongside a robust anti-correlation between ionization timescale and electron temperature, which is interpreted as evidence for ejecta clumping and a nuanced reverse-shock history. Nonthermal synchrotron emission dominates the 4–6 keV continuum in many regions, with a power-law fraction often exceeding 47%, a finding consistent with previous NuSTAR and IXPE results; no distinct Radiative Recombination Continuum features are detected in the current XRISM data. The work demonstrates XRISM’s capability for detailed SNR plasma mapping and highlights the need for full Cas A coverage and integrated multi-instrument analyses to fully capture the remnant’s complexity.

Abstract

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.

Figures (21)

• Figure 1: The XRISM/Resolve observations of Cas A are shown projected onto a three-color Chandra image (ObsID 4638) --- red: 6.50--6.75 keV (Fe K$\alpha$ complex); green: 1.76--1.94 keV (Si He$\alpha$); blue: 4.00--6.00 keV (continuum). The SE (ObsID 000129000) and NW (ObsID 000130000) XRISM pointing fields of view are shown in blue and red, respectively. The white dashed boxes outline the XRISM/Resolve pixels and the solid boxes show the 2$\times$2 pixel binnings ("super-pixels"), labeled per observation from a to i. The white '$\times$' symbol marks the expansion center from thorstensen01.
• Figure 2: XRISM/Resolve spectra in the 1.8--11.9 keV energy range from the central super-pixel e ($2\times2$ pixel) for the SE (left) and NW (right) pointing, with their corresponding fitted spectral model using UltraSPEX. The total model is shown in a solid red line and the model components are shown in dashed lines with the corresponding 1 sigma uncertainties marked in shaded bands. The orange, blue, yellow and green dashed line corresponds to the pure-metal IMEs pshock component, pure-metal Fe-group pshock component, the power-law component and the NXB component, respectively. The data are represented by the black data points and have been rebinned to minimum signal to noise ratio of 5 for display purposes only. The lower panels display the residuals between the data and the fitted model.
• Figure 3: The panels show the fitted abundance ratios to solar abundance ratios lodders09 for S/Si, Ar/Si, and Ca/Si (Si was fixed to 1e5 times solar value). The average $1\sigma$ error is indicated on the bottom right of each subplot. In all the figures with parameter maps, for the overlapping pixel regions near the center, an average parameter value is shown. The contours in grey show the outline of Cas A as derived from the Chandra observation from 2004 (ObsID 4638) using a broadband image.
• Figure 4: The fitted abundance ratio of Ni to Fe compared to the solar abundance ratio (Ni/Fe)$_{\odot}$. The average one sigma error for the fitted value is indicated on the bottom right.
• Figure 5: Top row: The fitted redshift maps for the IMEs (top left) and IGEs (top center) pshock components are shown. The map of difference in velocites (IGEs--IMEs) between the two is shown in the top right plot. Bottom row: The maps of fitted velocity dispersion measurements for the IMEs, IGEs and their difference (IGEs--IMEs) are presented left to right. All values are reported in units of km s$^{-1}$ and the average 1 sigma errors are marked at the bottom right of each plots.