Chandra identification of 26 new black hole candidates in the central region of M31

Abstract

We have previously identified 10 M31 black hole candidates (BHCs) in M31, from their X-ray properties alone. They exhibit "hard state" emission spectra that are seen at luminosities < ~10% Eddington in X-ray binaries (XBs) containing a neutron star (NS) or black hole (BH), at luminosities that significantly exceed the NS threshold. Nine of these are associated with globular clusters (GCs); hence, these are most likely low mass X-ray binaries (LMXBs); eight are included in this survey. We have recently discovered that analysis of the long term 0.5--4.5 keV variability of XBs via structure functions allows us to separate XBs from AGN, even though the emission spectra are often similar; this has enabled us to search for BHCs outside of GCs. We have identified 26 new BHCs (12 strong, 14 plausible) within 20' of the M31 nucleus (M31*), using 152 Chandra observations spaced over ~13 years; some of our classifications were enhanced with XMM-Newton observations. Of these, 7 appear within 100" of M31*; this supports the theory suggesting that this region experiences enhanced XB production via dynamical processes similar to those seen in GCs. We have found a parameter space where our black hole candidates are separated from Galactic neutron star binaries: we show that modelling a simulated hard state spectrum with a disk blackbody + blackbody model yields parameters that lie outside the space occupied by neutron star binaries that are modeled this way. The probability that our BHCs all lie within the NS parameter space is ~3E-29.