A NuSTAR View of the Hard X-ray Binary Population of the M31 Disk

Using hard X-ray observations attainable with NuSTAR, we are able to differentiate between the accretion states, and thus compact object types, of X-ray binaries (XRBs) in the Andromeda Galaxy, our nearest spiral neighbor. 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, as well as the population of potentially detectable gravitational wave sources. This tells us not only about the XRB population of the M31 disk, but also provides hints about the population and distribution of XRBs in the Milky Way, where this type of characterization is more difficult due to the ambiguity of the spatial distribution of sources. Using ten ~50ks observations of the disk of M31, we detect 19 XRB sources in the 4-25keV bandpass, and make classifications of 10 of those sources with the use of diagnostic color-rate and color-color diagrams, which separate sources into various neutron star and black hole regimes. We also investigate a candidate black hole associated with a globular cluster. We combine this with additional deeper observations of the disk to create X-ray luminosity functions for both the full (4-25keV) and hard (12-25keV) band luminosities. We find that they are consistent with the shapes of Milky Way XLFs by Voss & Ajello (2010), Lutovinov (2013), and Doroshenko (2014). We include fits of the XLFs of sub-populations of the characterized XRBs, including those in and outside of globular clusters and by compact object type. These classifications will be added to those already included in a deeper M31 survey of the bulge and a more limited coverage of the disk, expanding the knowledge of both the types of sources present in the population as well as their distribution throughout the galaxy.