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

CXOM31 J004252.030+413107.87 is one of the brightest X-ray sources within the D-25 region of M31, and associated with a globular cluster known as B135; we therefore call this X-ray source XB135. XB135 is a lowmass X-ray binary (LMXB) that apparently exhibited hard state characteristics at 0.3-10 keV luminosities 4-6 x 10(38) erg s(-1), and the hard state is only observed below similar to 10% Eddington. If true, the accretor would be a high-mass black hole (BH) (greater than or similar to 50 M-circle dot); such a BH may be formed from direct collapse of a metal-poor, high-mass star, and the very low metallicity of B135 (0.015 Z(circle dot)) 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 similar to 10-20 M-circle dot 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 (NS) LMXBs (known as Z-sources); however, we prefer a BH accretor. We note that deeper observation of XB135 could discriminate against an NS accretor.