Characterizing the Hard X-ray Binary Population of the Disk of M31

X-ray observations of nearby galaxies provide one of the best laboratories in the universe for studying the populations of two exotic classes of object: black holes and neutron stars. Because they are directly connected to past and current stellar populations through binary synthesis modeling, studies of accreting black hole and neutron star populations can provide insights into the history of star formation and evolution in a galaxy. Using the hard (E > 12 keV) X-ray sensitivity of NuSTAR, we are able to differentiate between the accretion states and compact object types of X-ray binaries (XRBs) in the Andromeda Galaxy (M31), the nearest Milky Way-mass galaxy to us. We combine 10 shallow (~50 ks each) observations across the disk of M31 with a previous deep, three-field (500 ks per field) survey of a smaller region of Andromeda's disk. The shallow survey adds 20 sources detected at 3-sigma in the 4-25 keV bandpass to the ~60 sources detected in the deep survey, crucially increasing the number of sources detected at higher luminosities. Using hardness diagrams that compare our source colors to those of known Milky Way XRB compact object type and states, we classify 12 (29) of these sources in the shallow (deep) surveys and contrast these results with that of M31's bulge, which is dominated by old (non-magnetized) neutron star accretors. In addition, we construct a luminosity function (LF) of sources detected in the 12-25 keV bandpass for the combined survey. This LF is directly compared to that of the high mass XRB LF found in the Milky Way through hard X-ray surveys conducted by Swift BAT and INTEGRAL, which are dominated by sources of similar (or lower) luminosity. At present, comparison of extragalactic XRBs with those of the Milky Way at these luminosities is only feasible for M31; we also discuss how a future hard X-ray observatory could revolutionize our understanding of XRB populations in the universe.