2011ApJ...734...79B

We have identified four new black hole candidates (BHCs) in M31 globular clusters (GCs) 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 observed luminosity, ∼1.4x 10³⁸ erg/s, considerably brighter than any outbursts from neutron star transients Aql X-1 or 4U 1608-452; the outburst apparently started ∼45 days earlier than the observed peak, and hence the luminosity may 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 ≲ 4x10^–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 four persistently bright BHCs in the region; the probability that these are all background active galactic nuclei is ≲ 1x10^–20. According to theory, the donors could be tidally captured main-sequence stars or white dwarfs in ultracompact binaries. We find that GCs that are particularly massive (XB082) or metal-rich (XB144) can host bright X-ray sources in addition to GCs that are both (XB163). Our method may reveal BHCs in other bright X-ray sources.