Asymmetric distribution of Fe-peak elements in Cassiopeia A revealed by XRISM

TL;DR

This study uses XRISM/Resolve to perform high-resolution spectroscopy of Cas A, revealing a highly asymmetric distribution of Fe-peak elements across three Fe-rich blobs. Mn is significantly enhanced in the SE region while being undetected in the NW, with Mn/Cr ratios differing markedly between regions; Ti is marginally detected in SE and Ni/Fe shows regional variation. The findings suggest a combination of differential mixing of Si-burning layers, spatial variations in the electron fraction $Y_e$, and neutrino irradiation (including the $ u$-process and $ u p$-process) shaping inner ejecta, rather than simple material mixing alone. Comparisons with two-dimensional neutrino-driven SN models indicate that the observed trends are consistent with proton-rich ejecta in some regions and neutron-rich or alpha-rich zones in others, highlighting the importance of multidimensional neutrino physics and progenitor structure in core-collapse supernova asymmetry. These results provide new constraints on asymmetric explosion mechanisms and motivate future 3D simulations and deeper XRISM mappings to quantify the roles of neutrino processing and mixing in early ejecta.

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 (CC SNe). In particular, the supernova remnant Cassiopeia 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 Cassiopeia 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 Cassiopeia 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.

Figures (4)

• Figure 1: Left: The field of view of the XRISM/Resolve pointings shown on the three-color image of Cas A taken by Chandra. Red, green, and blue include emission within energy bands of 6.54--6.92 keV (Fe He$\alpha$), 1.76-1.94 keV (Si He$\alpha$), and 4.2-6.0 keV (continuum), respectively. The dashed ellipses indicate the approximate locations of the three Fe-rich blobs in Cas A.
• Figure 2: Left: Two-color image of Cas A obtained with XRISM/Xtend and Chandra. Red indicates the 0.5–12 keV emission observed with Xtend, and green shows the Si-to-Mg line emission ratio map. The spectra were extracted from the white-box region labeled “JET.” Right: X-ray spectrum from the JET region (black points) along with the best-fit model (red solid line) in the 1.6–12 keV energy range. The lower panel displays the residuals between the data and the model. The orange, light blue, and blue solid lines correspond to the low-temperature plasma component, the high-temperature plasma component, and the power-law component, respectively.
• Figure 3: X-ray spectrum of the SE region obtained with Resolve (black points) together with the best-fit model (red solid line). The left panel presents the fit in the 1.6–12 keV energy range, while the lower panel shows the residuals between the data and the model. The orange, light blue, blue, and gray solid lines represent the low-temperature plasma model, the high-temperature plasma model, the power-law model, and the NXB model, respectively. The multiple panels on the right show zoomed-in views of the energy bands of Ti, Cr, Mn, and Ni.
• Figure 4: Top panels: Comparison of the elemental mass ratios in individual fluid elements of a 2D supernova model with those observed in Cas A. From left to right, the Mn/Cr versus Ca/Fe plot, the Mn/Cr versus Ni/Fe plot, and the Cr/Fe versus Ti/Fe plot are shown. Model points are colored by their electron fraction ($Y_e$), with light blue indicating low $Y_e$ regions and magenta indicating high $Y_e$ regions. Data points for $T_{\rm peak} > 5.5$ GK (i.e., complete Si burning or $\alpha$-rich freezeout) are represented by large circles, whereas those for $T_{\rm peak} < 5.5$ GK (i.e., incomplete Si burning or quasi-equilibrium: QSE) are represented by small circles. Observational values appear as red and blue data points with error bars (1$\sigma$ confidence level). In the Ti/Fe vs. Cr/Fe plot, the observational values reported in 2021Natur.592..537S are indicated by gray areas. Bottom panels: Same as the top panels, but compared with nucleosynthesis calculations with the electron fraction fixed at $Y_e = 0.51$, resembling a case where most of the ejecta become proton-rich ($Y_e > 0.5$) owing to neutrino irradiation. The electron fraction is fixed only for ejecta with peak temperatures exceeding 5.5 GK. The observational values in these plots are taken from Table \ref{['tab:best-fit']}.