  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.
