50 M31 black hole candidates identified by Chandra and XMM-Newton

TL;DR

This study identifies approximately 50 black hole candidates in M31 using X-ray spectral analysis from Chandra and XMM-Newton data, revealing their spectral states, duty cycles, and dominance in high-luminosity X-ray binaries.

Contribution

The paper presents a comprehensive identification and analysis of 50 black hole candidates in M31, including spectral state distinctions and an estimation of their prevalence among X-ray binaries.

Findings

42 strong BHCs identified

>40% of X-ray binaries contain black hole accretors

BHCs contribute >90% of high-luminosity X-ray binaries

Abstract

Over the last ~5 years we have identified ~35 black hole candidates (BHCs) in M31 from their X-ray spectra. Our BHCs exhibited 0.3--10 keV spectra consistent with the X-ray binary (XB) hard state, at luminosities that are above the upper limit for neutron star (NS) XBs. When our BHC spectra were modeled with a disk blackbody + blackbody model, for comparison with bright NS XBs, we found that the BHCs inhabited a different parameter space to the NS XBs. However, BH XBs may also exhibit a thermally dominated (TD) state that has never been seen in NS XBs; this TD state is most often observed in X-ray transients. We examined the ~50 X-ray transients in our Chandra survey of M31, and found 13 with spectra suitable for analysis. We also examined 2 BHCs outside the field of view of our survey, in the globular clusters B045 and B375. We have 42 strong BHCs, and 8 plausible BHCs that may benefit from further observation. Of our 15 BHCs in globular clusters, 12 differ from NS spectra by >5 sigma. Due to improvements in our analysis, we have upgraded 10 previously identified plausible BHCs to strong BHCs. The mean maximum duty cycle of the 33 X-ray transients within 6' of M31* was 0.13; we estimate that >40% of the XBs in this region contain BH accretors. Remarkably, we estimate that BHCs contribute >90% of those XBs >1 E+38 erg/s.