Four new black hole candidates identified in M31 globular clusters with Chandra and XMM-Newton

TL;DR

This study reports the discovery of four new black hole candidates in M31 globular clusters using Chandra and XMM-Newton data, with spectral and variability analysis supporting their black hole nature.

Contribution

It introduces a method combining spectral and variability criteria to identify black hole candidates in extragalactic globular clusters, expanding the known population.

Findings

Four new black hole candidates identified in M31 globular clusters.

Spectral and variability analysis supports black hole classification.

Probability estimates strongly favor black hole over neutron star or background sources.

Abstract

We have identified four new black hole candidates in M31 globular clusters using 123 Chandra, and 4 XMM-Newton observations of the M31 central region. The X-ray source associated with Bo 163 (XB163) is a recurrent transient, with the highest luminosity ~1.4E+38 erg/s, considerably brighter than any outbursts from the neutron star transients Aql X-1 or 4U 1608-452; the outburst apparently started ~45 days earlier than the observed peak, hence the luminosity could have been considerably higher. We identified XB082, XB153 and XB185 as BHCs by observing "low state" emission spectra at luminosities that exceed the threshold for neutron star binaries. The probability that these are neutron star systems with anisotropic emission beamed toward us is <4E-4, and their variability suggests emission from a single source. We therefore conclude that these systems likely contain black holes rather than neutron stars. We have now identified 4 persistently bright BHCs in the region; the probability that these are all background AGN is <1E-20. According to theory, the donors could be tidally captured main sequence stars, or white dwarves in ultra-compact binaries. We find that GCs that are particularly massive (XB082) or metal rich (XB144) can host bright X-ray sources in addition to those that are both (XB163). Our method may reveal BHCs in other bright X-ray sources.