A NuSTAR Census of the X-Ray Binary Population of the M31 Disk

Abstract

Using hard (E > 10 keV) X-ray observations with NuSTAR, we are able to differentiate between the accretion states, and thus compact object types, of neutron stars (NSs) and black holes (BHs) in X-ray binaries (XRBs) in M31, our nearest Milky Way–type neighbor. Using 10 moderate-depth (20–50 ks) observations of the disk of M31 covering a total of ~0.45 deg², we detect 20 sources at 2σ in the 4–25 keV bandpass, 14 of which we consider to be XRB candidates. This complements an existing, deeper (100–400 ks) survey covering ~0.2 deg² of the bulge and the northeastern disk. We make tentative classifications of nine of these sources with the use of diagnostic color–intensity and color–color diagrams, which separate sources into various NS and BH regimes, identifying three BHs and six NSs. In addition, we create X-ray luminosity functions (XLFs) for both the full (4–25 keV) and hard (12–25 keV) bands, as well as subpopulations of the full band based on compact object type and association with globular clusters. Our best-fit globular cluster XLF is shallower than the field XLF, and preliminary BH and NS XLFs suggest a difference in shape based on compact object type. We find that the cumulative disk XLFs in the full and hard band are best fit by power laws with indices of 1.32 and 1.28, respectively. This is consistent with models of the Milky Way XLF from Grimm et al., Voss & Ajello, and Doroshenko et al.