A systematic study of low mass X-ray binaries in the M31 globular cluster system

TL;DR

This study provides a comprehensive analysis of low mass X-ray binaries in M31's globular clusters, revealing correlations with cluster properties and emphasizing the role of stellar collision rates in LMXB formation.

Contribution

It offers the most complete X-ray survey of M31's globular clusters to date and clarifies the relationship between LMXB presence and cluster characteristics.

Findings

45 clusters in M31 are associated with X-ray emission.

LMXB presence correlates with metallicity, luminosity, and stellar collision rate.

Stellar collision rate is the key factor in LMXB formation.

Abstract

We investigate low mass X-ray binaries (LMXBs) in the M31 globular cluster (GC) system using data from the 2XMMi catalogue. These X-ray data are based on all publicly available XMM-Newton observations of the galaxy. This new survey provides the most complete and homogeneous X-ray survey of M31's GCs to date, covering >80% of the confirmed old clusters in the galaxy. We associate 41 X-ray sources with confirmed old clusters in the M31 cluster catalogue of Peacock et al. (2010). Comparing these data with previous surveys of M31, it is found that three of these clusters are newly identified, including a bright transient source in the cluster B128. Four additional clusters, that are not detected in these 2XMMi data, have previously been associated with X-ray sources from Chandra or ROSAT observations. Including these clusters, we identify 45 clusters in M31 which are associated with X-ray emission. By considering the latest optical GC catalogues, we identify that three of the previously proposed X-ray clusters are likely to be background galaxies and two have stellar profiles. We consider the properties of LMXB-hosting clusters and confirm significant trends between the presence of an LMXB and the metallicity, luminosity and stellar collision rate of a cluster. We consider the relationship between the luminosity and stellar collision rate of a cluster and note that LMXB hosting clusters have higher than average stellar collision rates for their mass. Our findings strongly suggest that the stellar collision rate is the dominant parameter related to the presence of LMXBs. This is consistent with the formation of LMXBs in GCs through dynamical interactions.