Unraveling the Origin of Overionized Plasma in the Galactic Supernova Remnant W49B

TL;DR

This study maps overionized plasma in the supernova remnant W49B, revealing a west-to-east gradient likely caused by adiabatic expansion cooling, advancing understanding of plasma cooling mechanisms in SNRs.

Contribution

The paper provides the first spatially-resolved mapping of overionized plasma in W49B, linking overionization to adiabatic expansion cooling.

Findings

Overionization is concentrated in the west of W49B.

A gradient of increasing overionization from east to west was observed.

Adiabatic expansion is identified as the primary cooling mechanism.

Abstract

Recent observations have shown several supernova remnants (SNRs) have overionized plasmas, those where ions are stripped of more electrons than they would be if in equilibrium with the electron temperature. Rapid electron cooling is necessary to produce this situation, yet the physical origin of that cooling remains uncertain. To assess the cooling scenario responsible for overionization, in this paper, we identify and map the overionized plasma in the Galactic SNR W49B based on a 220 ks Chandra Advanced CCD Imaging Spectrometer (ACIS) observation. We performed a spatially-resolved spectroscopic analysis, measuring the electron temperature by modeling the continuum and comparing it to the temperature given by the flux ratio of the He-like and H-like lines of sulfur and of argon. Using these results, we find that W49B is overionized in the west, with a gradient of increasing overionization from east to west. As the ejecta expansion is impeded by molecular material in the east but not in the west, our overionization maps suggest the dominant cooling mechanism is adiabatic expansion of the hot plasma.