Faint X-ray Binaries and Their Optical Counterparts in M31
  X-ray binaries (XRBs) are probes of both star formation and stellar mass, but
more importantly remain one of the only direct tracers of the compact object
population. To investigate the XRB population in M31, we utilized all 121
publicly available observations of M31 totalling over 1 Ms from
$\it{Chandra's}$ ACIS instrument. We studied 83 star clusters in the bulge
using the year 1 star cluster catalogue from the Panchromatic Hubble Andromeda
Treasury Survey. We found 15 unique star clusters that matched to 17 X-ray
point sources within 1'' (3.8 pc). This population is composed predominantly of
globular cluster low-mass XRBs, with one previously unidentified star cluster
X-ray source. Star clusters that were brighter and more compact preferentially
hosted an X-ray source. Specifically, logistic regression showed that the F475W
magnitude was the most important predictor followed by the effective radius,
while color (F475W$-$F814W) was not statistically significant. We also
completed a matching analysis of 1566 HII regions and found 10 unique matches
to 9 X-ray point sources within 3'' (11 pc). The HII regions hosting X-ray
point sources were on average more compact than unmatched HII regions, but
logistic regression concluded that neither the radius nor H$\alpha$ luminosity
was a significant predictor. Four matches have no previous classification and
thus are high-mass XRB candidates. A stacking analysis of both star clusters
and HII regions resulted in non-detections, giving typical upper limits of
$\approx10^{32}$ erg s$^{-1}$, which probes the quiescent XRB regime.
