Barnard, R.; Stiele, H.; Hatzidimitriou, D.; Kong, A. K. H.; Williams, B. F.; Pietsch, W.; Kolb, U. C.; Haberl, F. and Sala, G.
New XMM-Newton Analysis of Three Bright X-Ray Sources in M31 Globular Clusters, Including a New Black Hole Candidate.
Astrophysical Journal, 689(2) pp. 1215–1221.
We present a 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 light curve was seen to vary by 10%, and its 0.3–7.0 keV emission spectrum was well described by an absorbed power law with photon index . 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 behavior is typically observed at luminosities 10% Eddington. However, XBo 45 exhibited this behavior at an unabsorbed, 0.3–10 keV luminosity of erg s−1, or 140% Eddington for a 1.4 neutron star accreting hydrogen. Hence, we identify XBo 45 as a new candidate black hole LMXB. XBo 45 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 a neutron star.
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