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arXiv · 0807.0128

New XMM-Newton analysis of three bright X-ray sources in M31 globular clusters, including a new black hole candidate

R. Barnard·H. Stiele·D. Hatzidimitriou·A. K. H. Kong·B. F. Williams·W. Pietsch·U. C. Kolb·F. Haberl·G. Sala
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Abstract

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$\pm$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 $\la$10% Eddington. However, XBo 45 exhibited this behaviour at an unabsorbed, 0.3--10 keV luminosity of 2.5$\pm0.2\times 10^{38}$ erg s$^{-1}$, or{~140%} Eddington for a 1.4 $M_{\odot}$ 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 neutron star.

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R. Barnard, H. Stiele, D. Hatzidimitriou, A. K. H. Kong, B. F. Williams, W. Pietsch, U. C. Kolb, F. Haberl, G. Sala. 2008-07-01. New XMM-Newton analysis of three bright X-ray sources in M31 globular clusters, including a new black hole candidate. https://doi.org/10.1086/592594

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