Asymmetric Distribution of Fe-peak Elements in Cassiopeia A Revealed by XRISM

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.