A Decade-Baseline Study of the Plasma States of Ejecta Knots in Cassiopeia A

TL;DR

This study analyzes the plasma states of ejecta knots in Cassiopeia A over a decade, revealing discrepancies in temperature evolution and proposing a dissociated ejecta model to explain the observations.

Contribution

It provides one of the first comparisons between high-resolution and ACIS-derived plasma temperatures in supernova remnant knots, and introduces a dissociated ejecta model based on observational inconsistencies.

Findings

ACIS and HETG temperature estimates agree for about half of the knots.

Most knots do not show the expected spectral evolution from ionization age and density.

A dissociated ejecta model is proposed to explain the observations.

Abstract

We present the analysis of 21 bright X-ray knots in the Cassiopeia A supernova remnant from observations spanning 10 yr. We performed a comprehensive set of measurements to reveal the kinematic and thermal state of the plasma in each knot, using a combined analysis of two high energy resolution High Energy Transmission Grating (HETG) and four medium energy resolution Advanced CCD Imaging Spectrometer (ACIS) sets of spectra. The ACIS electron temperature estimates agree with the HETG-derived values for approximately half of the knots studied, yielding one of the first comparisons between high resolution temperature estimates and ACIS-derived temperatures. We did not observe the expected spectral evolutionpredicted from the ionization age and density estimates for each knotin all but three of the knots studied. The incompatibility of these measurements with our assumptions has led us to propose a dissociated ejecta model, with the metals unmixed inside the knots, which could place strong constraints on supernova mixing models.