  We present 15 high mass X-ray binary (HMXB) candidates in the disk of M31 for
which we are able to infer compact object type, spectral type of the donor
star, and age using multiwavelength observations from NuSTAR, Chandra, and the
Hubble Space Telescope (HST). The hard X-ray colors and luminosities from
NuSTAR permit the tentative classification of accreting X-ray binary systems by
compact object type, distinguishing black hole from neutron star systems. We
find hard state black holes, pulsars, and non-magnetized neutron stars
associated with optical point source counterparts with similar frequency. We
also find nine non-magnetized neutron stars coincident with globular clusters
and an equal number of pulsars with and without point source optical
counterparts. We perform spectral energy distribution (SED) fitting for the
most likely optical counterparts to the HMXB candidates, finding 7 likely high
mass stars and 1 possible red Helium burning star. The remaining 7 HMXB optical
counterparts have poor SED fits, so their companion stars remain unclassified.
Using published star formation histories, we find that the majority of HMXB
candidates --- X-ray sources with UV-bright point source optical counterpart
candidates --- are found in regions with star formation bursts less than 50 Myr
ago, with 3 associated with young stellar ages (<10 Myr). This is consistent
with similar studies of HMXB populations in the Magellanic Clouds, M33, NGC
300, and NGC 2403.
