Three-Dimensional Expansion of Iron Ejecta in Cassiopeia A with Chandra and XRISM

Abstract

Measurement of the expansion structure of ejecta in supernova remnants (SNRs) provides important information about their explosion mechanism. We combine Chandra proper-motion measurements with XRISM Doppler measurements to derive an approximate, region-averaged three-dimensional velocity map of the Fe-K-emitting ejecta in Cassiopeia A (Cas A), one of the youngest Galactic core-collapse SNRs. Proper motions were measured using the optical flow method. We find that the Fe-K-emitting ejecta expand predominantly outward, except in the western region, with projected velocities of up to ~4580 km/s. The southeastern ejecta exhibit the highest three-dimensional velocity, reaching 5160+-320 km/s, based on the combined proper-motion and Doppler measurements. This velocity exceeds those measured for intermediate-mass elements (IMEs) such as Si and S, and the corresponding ejecta exhibit a relatively small line-of-sight velocity dispersion. These results are broadly consistent with the previously suggested inversion between the Fe-rich and IME-rich ejecta layers. The apparent inward motion or emergence of a new Fe-K emission component in the western region may reflect interaction with dense surrounding material, although the present data do not allow us to distinguish uniquely between these possibilities or to determine their physical origin.