A second black hole candidate in a M31 globular cluster is identified with XMM-Newton

Barnard, R. and Kolb, U. (2009). A second black hole candidate in a M31 globular cluster is identified with XMM-Newton. Monthly Notices of the Royal Astronomical Society, 397 L92-L95.

DOI: https://doi.org/10.1111/j.1745-3933.2009.00691.x

Abstract

We use arguments developed in previous work to identify a second black hole candidate associated with an M31 globular cluster (GC), Bo 144, on the basis of X-ray spectral and timing properties. The 2002 XMM–Newton observation of the associated X-ray source (hereafter XBo 144) revealed behaviour that is common to all low-mass X-ray binaries (LMXBs) in the low-hard state. Studies have shown that neutron star LMXBs exhibit this behaviour at 0.01–1000 keV luminosities ≤10 per cent of the Eddington limit (LEdd). However, the unabsorbed 0.3–10 keV luminosity of XBo 144 was ~0.30 LEdd for a 1.4 M⊚ neutron star, and the 0.01–1000 keV luminosity is expected to be ≳3–7 times higher. We therefore identify XBo 144 as a black hole candidate. Furthermore, it is the second black hole candidate to be consistent with formation via tidal capture of a main sequence donor in a GC; such systems were previously thought non-existent, because the donor was thought to be disrupted during the capture process.

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