Spatially-Resolved Study of Recombining Plasma in W49B Using XMM-Newton

TL;DR

This study uses spatially-resolved XMM-Newton data to analyze recombining plasma in W49B, revealing overionization across the entire remnant and suggesting different cooling mechanisms in different regions.

Contribution

It provides the first spatially-resolved spectroscopic analysis of W49B, showing overionization in both east and west, and evaluates thermal conduction as a cooling process.

Findings

Overionization is present throughout W49B.

Overionization increases from east to west.

Thermal conduction via cloud evaporation can explain east overionization.

Abstract

W49B is the youngest SNR to date that exhibits recombining plasma. The two prevailing theories of this overionization are rapid cooling via adiabatic expansion or through thermal conduction with an adjacent cooler medium. To constrain the origin of the recombining plasma in W49B, we perform a spatially-resolved spectroscopic study of deep XMM-Newton data across 46 regions. We adopt a 3-component model (with one ISM and two ejecta components), and we find that recombining plasma is present throughout the entire SNR, with increasing overionization from east to west. The latter result is consistent with previous studies, and we attribute the overionization in the west to adiabatic expansion. However, our findings contrast these prior works as we find evidence of overionization in the east as well. As the SNR is interacting with molecular material there, we investigate the plausibility of thermal conduction as the origin of the rapid cooling. We show that based on the estimated timescales, it is possible that small-scale thermal conduction through evaporation of clumpy, dense clouds with a scale of 0.1-1.0 pc can explain the observed overionization in the east.