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

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CXOM31 J004252.030+413107.87 is one of the brightest X-ray sources within the D₂5 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 keV luminosities 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) (> 50 Mₛun); such a BH may be formed from direct collapse of a metal-poor, high mass star, and the very low metalicity of B135 (0.015 Zₛun) makes such a scenario plausible. We have obtained new XMM-Newton and Chandra HRC observations to shed light on the nature of this object. We find from the HRC observation that XB135 is a single point source located close to the center of B135. The new XMM-Newton spectrum is consistent with a rapidly spinning 10--20 Mₛun BH in the steep power law or thermal dominant state, but inconsistent with the hard state that we previously assumed. We cannot formally reject three component emission models that have been associated with high luminosity neutron star LMXBs (known as Z-sources); however, we prefer a BH accretor. We note that deeper observation of XB135 could discriminate against a neutron star accretor.