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M31球状星团中三个明亮X射线源的新XMM-牛顿分析，包括一个新的黑洞候选者

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

备注：
Accepted by ApJ, 16 pages, 5 figures. This version includes the final changes made at the request of the referee
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摘要：

本文详细分析了M31的XMM-牛顿扩展巡天中的三个球状星团X射线源。M31球状星团Bo 45（XBo 45）的X射线对应物在2006年12月26日被XMM-牛顿观测到。其组合pn+MOS 0.3--10 keV光变曲线的均方根变化率约为10%，其0.3--7.0 keV发射光谱符合吸收功率定律，光子指数为1.44 $\pm$ 0.12。无论吸积体是中子星还是黑洞，它的变化和发射都是低硬态低质量X射线双星的特征。这种行为通常在亮度$\la$ 10%爱丁顿下观察到。然而，XBo 45在未吸收的0.3- 10 keV光度为2.5 $\pm0.2\times 10^{38}$ erg s $^{-1}$或{~140%} Eddington（对于吸积氢的1.4 $M_{\odot}$中子星）时表现出这种行为。因此，我们确定XBo 45为一个新的候选黑洞LMXB。XBo 45似乎已经持续明亮了30年，这与理论预测的通过潮汐捕获形成的球状星团黑洞双星一致。Bo 375是在2007年1月2日的XMM-牛顿观测中观测到的，它有一个典型的明亮中子星LMXB的双组分光谱。Bo 135与Bo 45在同一个区域被观测到，可能包含一个黑洞或中子星。

英文摘要：

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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