A Study of Radio Knots within Supernova Remnant Cassiopeia A

TL;DR

This paper analyzes the proper motion, brightness changes, and acceleration of 260 radio knots in supernova remnant Cassiopeia A, revealing insights into the remnant's density structure and magnetic field evolution.

Contribution

It provides detailed measurements of radio knot dynamics in Cas A, highlighting the prevalence of deceleration and rapid magnetic field changes, which were not previously quantified.

Findings

75% of knots are decelerating, indicating density fluctuations in stellar winds

Knot acceleration/deceleration is unrelated to brightness distribution

Rapid magnetic field changes inferred from brightness variability

Abstract

The study on the dynamic evolution of young supernova remnants (SNRs) is an important way to understand the density structure of the progenitor's circumstellar medium. We have reported the acceleration or deceleration, proper motion and brightness changes of 260 compact radio features in the second youngest known SNR Cas A at 5\,GHz based on the VLA data of five epochs from 1987 to 2004. The radio expansion center locates at $\alpha(1950)=23^{\rm h}21^{\rm m}9^{\rm s}_{\cdot}7 \pm 0^{\rm s}_{\cdot}29, \delta(1950)=+58^{\circ}32^{\prime}25^{\prime\prime}_{\cdot}2 \pm 2^{\prime\prime}_{\cdot}2$. Three-quarters of the compact knots are decelerating, this suggests that there are significant density fluctuations in the stellar winds of the remnant's progenitor. We have verified that the acceleration or deceleration of compact knots are not related with the distribution of brightness. The brightening, fading, disappearing or new appearing of compact radio features in Cas A suggests that the magnetic field in the remnant is changing rapidly.