New Xmm-newton Analysis of Three Bright X-ray Sources in M31 Globular Clusters, including a New Black Hole Candidate

We present detailed analysis of three globular cluster X-ray sources in the XMM-Newton extended survey of M31. The X-ray counterpart to the M31 globular cluster Bo 45 (XBo 45) was observed with XMM-Newton on 2006 December 26. Its combined pn+MOS 0.3–10 keV lightcurve exhibited a r.m.s variability of ∼10%, and its 0.3–7.0 keV emission spectrum was well described by an absorbed power law with photon index 1.44±0.12. Its variability and emission is characteristic of low mass X-ray binaries (LMXBs) in the low-hard state, whether the accretor is a neutron star or black hole. Such behaviour is typically observed at luminosities .10% Eddington. However, XBo 45 exhibited this behaviour at an unabsorbed, 0.3–10 keV luminosity of 2.5±0.2×10 erg s, or ∼140% Eddington for a 1.4 M⊙ neutron star accreting hydrogen. Hence, we identify XBo 45 as a new candidate black hole LMXB. XBo45 appears to have been consistently bright for ∼30 years, consistent with theoretical prediction for a globular cluster black hole binary formed via tidal capture. Bo 375 was observed in the 2007, January 2 XMM-Newton observation, and has a two-component spectrum that is typical for a bright neutron star LMXB. Bo 135 was observed in the same field as Bo 45, and could contain either a black hole or neutron star. Subject headings: X-rays: binaries — galaxies: individual: M31 — methods: data analysis