Black Holes and Neutron Stars in Nearby Galaxies: Insights with NuSTAR

There are a handful of diagnostics that permit determination of compact object identity in X-ray binaries (XRBs), and most of these are confined to bright Galactic sources for which a large number of photons can be gathered. We report on recent work using sensitive hard X-ray constraints to separate black holes from neutron stars in external galaxies with NuSTAR. Determining the ratio of XRBs that are black holes or neutron stars in different galactic environments reveals critical clues about the formation and evolution of binary systems. We analyze a NuSTAR-selected sample of ≈10 nearby galaxies within 5 Mpc that represent a range of star formation rates (0.1 - 10 M_⊙ yr^-1) and stellar masses (10^9-11 M_⊙). Using color-color and color-intensity diagnostics we classify sources by their accretion states and compact object types. We analyze the 12-25 keV X-ray luminosity functions (XLFs) of our sample scaled by specific star formation rate and compare with the 0.5-8 keV analogues. Our diagnostic methods allow us to produce black hole-only and neutron star-only extragalactic XLFs for the first time.