A Comparative Study of the Supernova Remnant Cassiopeia A from 2013--2020 Deep [Fe II]+[Si I] Images

TL;DR

This study compares deep [Fe II]+[Si I] images of Cassiopeia A from 2013 and 2020, revealing morphological and kinematic changes over seven years, including flux fluctuations and proper motions of ejecta knots.

Contribution

It provides the first direct comparison of Cassiopeia A's remnant structure over seven years using identical observational setups, enabling precise measurements of knot motions and flux variations.

Findings

Most FMKs follow ballistic expansion.

Some FMKs show significant deceleration.

The ejecta shell's proper motion fits a uniformly expanding model.

Abstract

We present a comparative analysis of supernova remnant Cassiopeia A based on two deep, narrow-band images covering the [Fe II] 1.644um + [Si I] 1.645um lines obtained in 2013 and 2020 with the same instruments on the UKIRT 3.8m telescope. The identical setup and observing procedure allow for direct, accurate measurements of morphological and kinematic changes over a seven-year baseline. We identified 263 compact knots in the 2020 image and, through comparison with the 2013 catalog of Koo et al. 2018 (arXiv:1809.07935), classified them into quasi-stationary circumstellar knots and fast-moving knots (FMKs) of supernova ejecta. The FMKs show significant flux fluctuations, and many of those detected in 2013 are absent in the 2020 image. Proper-motion measurements derived from cross-correlation analysis indicate that most FMKs follow nearly ballistic expansion, whereas some, particularly those just beyond the eastern Fe-rich, X-ray emitting ejecta region, exhibit noticeable deceleration. The proper motions of the main ejecta shell were also measured and modeled as a uniformly expanding shell with a systemic motion, which reproduces the observed geometric and kinematic asymmetries of the remnant.