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 ( 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 \times 10 ^ { 38 } erg s ^ { -1 } , and the hard state is only observed below \sim 10 % Eddington .
If true , the accretor would be a high mass black hole ( BH ) ( \ga 50 M _ { \odot } ) ; 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 Z _ { \odot } ) 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 \sim 10–20 M _ { \odot } BH in the steep power law or thermal dominant state , but inconsistent with the hard state that we previously assumed .
We can not 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 .