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CXOM31 J004252.030+413107.87 is one of the brightest X-ray sources within theD_25 region of M31, and associated with a globular cluster (GC) known as B135;we therefore call this X-ray source XB135. XB135 is a low mass X-ray binary(LMXB) that apparently exhibited hard state characteristics at 0.3--10 keVluminosities 4--6 E+38 erg/s, and the hard state is only observed below ~10%Eddington. If true, the accretor would be a high mass black hole (BH) (> ~50M_Sun); such a BH may be formed from direct collapse of a metal-poor, high massstar, and the very low metalicity of B135 (0.015 Z_Sun) makes such a scenarioplausible. We have obtained new XMM-Newton and Chandra HRC observations to shedlight on the nature of this object. We find from the HRC observation that XB135is a single point source located close to the center of B135. The newXMM-Newton spectrum is consistent with a rapidly spinning ~10--20 M_Sun BH inthe steep power law or thermal dominant state, but inconsistent with the hardstate that we previously assumed. We cannot formally reject three componentemission models that have been associated with high luminosity neutron starLMXBs (known as Z-sources); however, we prefer a BH accretor. We note thatdeeper observation of XB135 could discriminate against a neutron star accretor.

XMM-Newton and Chandra observations of the M31 globular cluster black hole candidate XB135: a heavyweight contender cut down to size