New XMM-Newton analysis of three bright X-ray sources in M31 globular clusters, including a new black hole candidate

TL;DR

This paper analyzes three bright X-ray sources in M31 globular clusters, identifying a new black hole candidate based on spectral and luminosity characteristics, and provides insights into their accretion states and nature.

Contribution

It presents detailed XMM-Newton analysis of three M31 globular cluster X-ray sources, including the first identification of a new black hole candidate in a globular cluster.

Findings

XBo 45 is a new black hole candidate based on luminosity and spectral analysis.

Bo 375 is identified as a neutron star low-mass X-ray binary.

Bo 135 could contain either a black hole or neutron star.

Abstract

We present detailed analysis of three globular cluster X-ray sources in the XMM-Newton extended survey of M31. The X-ray counterpart to the M31 globular cluster Bo 45 (XBo 45) was observed with XMM-Newton on 2006 December 26. Its combined pn+MOS 0.3--10 keV lightcurve exhibited a r.m.s variability of ~10%, and its 0.3--7.0 keV emission spectrum was well described by an absorbed power law with photon index 1.44$\pm$0.12. Its variability and emission is characteristic of low mass X-ray binaries (LMXBs) in the low-hard state, whether the accretor is a neutron star or black hole. Such behaviour is typically observed at luminosities $\la$10% Eddington. However, XBo 45 exhibited this behaviour at an unabsorbed, 0.3--10 keV luminosity of 2.5$\pm0.2\times 10^{38}$ erg s$^{-1}$, or{~140%} Eddington for a 1.4 $M_{\odot}$ neutron star accreting hydrogen. Hence, we identify XBo 45 as a new candidate black hole LMXB. XBo 45 appears to have been consistently bright for ~30 years, consistent with theoretical prediction for a globular cluster black hole binary formed via tidal capture. Bo 375 was observed in the 2007, January 2 XMM-Newton observation, and has a two-component spectrum that is typical for a bright neutron star LMXB. Bo 135 was observed in the same field as Bo 45, and could contain either a black hole or neutron star.