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

Abstract

We use arguments developed in previous work to identify a second black hole candidate associated with a M31 globular cluster, 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% of the Eddington limit (L_Edd). However, the unabsorbed 0.3-10 keV XBo 144 luminosity was ~0.30 L_Edd for a 1.4 M_sun neutron star, and the expected 0.01-1000 keV luminosity is 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 mean sequence donor in a GC; such systems were previously though non-existent, because the donor was thought to be disrupted during the capture process.