X-ray Studies of the Inverted Ejecta Layers in the Southeast Area of Cassiopeia A

TL;DR

This study uses X-ray data from Chandra to analyze the inverted ejecta layers in Cassiopeia A, revealing velocity differences and nucleosynthetic signatures that support early overturning of supernova ejecta.

Contribution

It provides detailed 3D velocity measurements and nucleosynthetic analysis of inverted ejecta layers, offering new insights into supernova explosion dynamics.

Findings

Fe-rich ejecta velocity >4500 km/s

Si/O-rich ejecta velocity ~2000-3000 km/s

Cr/Fe ratios indicate inverted burning layers

Abstract

The central strong activities in core-collapse supernovae expect to produce the overturning of the Fe- and Si/O-rich ejecta during the supernova explosion based on multi-dimensional simulations. X-ray observations of the supernova remnant Cassiopeia A have indicated that the Fe-rich ejecta lies outside the Si-rich materials in the southeastern region, which is consistent with the hypothesis on the inversion of the ejecta. We investigate the kinematic and nucleosynthetic properties of the inverted ejecta layers in detail to understand its formation process using the data taken by the Chandra X-ray Observatory. Three-dimensional velocities of Fe- and Si/O-rich ejecta are obtained as $>$4,500 km s$^{-1}$ and $\sim$2,000--3,000 km s$^{-1}$, respectively, by combining proper motion and line-of-sight velocities, indicating that the velocity of the Si/O-rich ejecta is slower than that of the Fe-rich ejecta since the early stage of the explosion. To constrain their burning regime, the Cr/Fe mass ratios are evaluated as $0.51_{-0.10}^{+0.11}$% in the outermost Fe-rich region and $1.24 ^{+0.19}_{-0.20}$% in the inner Fe/Si-rich region, suggesting that the complete Si burning layer is invertedly located to the incomplete Si burning layer. All the results support the ejecta overturning at the early stages of the remnant's evolution or during the supernova explosion of Cassiopeia A.