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

TL;DR

This study uses XMM-Newton and Chandra observations to analyze the nature of XB135, a bright X-ray source in M31's globular cluster, revealing it is likely a ~10-20 solar mass black hole rather than a neutron star.

Contribution

The paper provides new X-ray spectral data that suggest XB135 is a stellar-mass black hole, challenging previous assumptions of a high-mass black hole in a globular cluster.

Findings

XB135 is a single point source near the center of B135.

Spectral analysis indicates a ~10-20 M_Sun black hole in a specific accretion state.

The data are inconsistent with the previously assumed hard state, favoring a black hole interpretation.

Abstract

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 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 M_Sun); 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_Sun) 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 M_Sun 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.