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

Abstract

We investigate low mass X-ray binaries (LMXBs) in the M31 globular cluster\n(GC) system using data from the 2XMMi catalogue. These X-ray data are based on\nall publicly available XMM-Newton observations of the galaxy. This new survey\nprovides the most complete and homogeneous X-ray survey of M31's GCs to date,\ncovering >80% of the confirmed old clusters in the galaxy. We associate 41\nX-ray sources with confirmed old clusters in the M31 cluster catalogue of\nPeacock et al. (2010). Comparing these data with previous surveys of M31, it is\nfound that three of these clusters are newly identified, including a bright\ntransient source in the cluster B128. Four additional clusters, that are not\ndetected in these 2XMMi data, have previously been associated with X-ray\nsources from Chandra or ROSAT observations. Including these clusters, we\nidentify 45 clusters in M31 which are associated with X-ray emission. By\nconsidering the latest optical GC catalogues, we identify that three of the\npreviously proposed X-ray clusters are likely to be background galaxies and two\nhave stellar profiles. We consider the properties of LMXB-hosting clusters and\nconfirm significant trends between the presence of an LMXB and the metallicity,\nluminosity and stellar collision rate of a cluster. We consider the\nrelationship between the luminosity and stellar collision rate of a cluster and\nnote that LMXB hosting clusters have higher than average stellar collision\nrates for their mass. Our findings strongly suggest that the stellar collision\nrate is the dominant parameter related to the presence of LMXBs. This is\nconsistent with the formation of LMXBs in GCs through dynamical interactions.\n