Diffuse Thermal X-Ray Emission in the Core of the Young Massive Cluster Westerlund 1

TL;DR

This study analyzes the diffuse X-ray emission in Westerlund 1's core, revealing that the hard X-ray component is mainly thermal, likely from a cluster wind and pre-main sequence stars, with minimal supernova remnant contribution.

Contribution

The paper clarifies the thermal nature of the hard X-ray emission in Westerlund 1 using XMM-Newton data, resolving previous ambiguities and identifying likely sources.

Findings

Hard X-ray emission is predominantly thermal, indicated by Fe 6.7 keV line.

Cluster wind is the main contributor to the hard X-ray emission.

Supernova remnants are unlikely to significantly contribute at this epoch.

Abstract

We present an analysis of the diffuse hard X-ray emission in the core of the young massive Galactic cluster Westerlund 1 based on a 48 ks XMM-Newton observation. Chandra results for the diffuse X-ray emission have indicated a soft thermal component together with a hard component that could be either thermal or non-thermal. We seek to resolve this ambiguity regarding the hard component exploiting the higher sensitivity of XMM-Newton to diffuse emission. Our new X-ray spectra from the central (2' radius) diffuse emission are found to exhibit He-like Fe 6.7 keV line emission, demonstrating that the hard emission in the cluster core is predominantly thermal in origin. Potential sources of this hard component are reviewed, namely an unresolved Pre-Main Sequence population, a thermalized cluster wind and Supernova Remnants interacting with stellar winds. We find that the thermalized cluster wind likely contributes the majority of the hard emission with some contribution from the Pre-Main Sequence population. It is unlikely that Supernova Remnants are contributing significantly to the Wd1 diffuse emission at the current epoch.