Black Holes and Neutron Stars in Nearby Galaxies: Insights from NuSTAR
  Nearby galaxy surveys have long classified X-ray binaries (XRBs) by the mass
category of their donor stars (high-mass and low-mass). The NuSTAR observatory,
which provides imaging data at E $>10$ keV, has enabled the classification of
extragalactic XRBs by their compact object type: neutron star (NS) or black
hole (BH). We analyzed NuSTAR/Chandra/XMM-Newton observations from a
NuSTAR-selected sample of 12 galaxies within 5 Mpc having stellar masses
($M_{\star}$) $10^{7-11}$ $M_{\odot}$ and star formation rates (SFR)
$\approx0.01-15$ $M_{\odot}$ yr$^{-1}$. We detect 128 NuSTAR sources to a
sensitivity of $\approx10^{38}$ erg s$^{-1}$. Using NuSTAR color-intensity and
color-color diagrams we classify 43 of these sources as candidate NS and 47 as
candidate BH. We further subdivide BH by accretion states (soft, intermediate,
and hard) and NS by weak (Z/Atoll) and strong (accreting pulsar) magnetic
field. Using 8 normal (Milky Way-type) galaxies in the sample, we confirm the
relation between SFR and galaxy X-ray point source luminosity in the 4-25 and
12-25 keV energy bands. We also constrain galaxy X-ray point source luminosity
using the relation $L_{\rm{X}}=\alpha M_{\star}+\beta\text{SFR}$, finding
agreement with previous work. The XLF of all sources in the 4-25 and 12-25 keV
energy bands matches with the $\alpha=1.6$ slope for high-mass XRBs. We find
that NS XLFs suggest a decline beginning at the Eddington limit for a 1.4
$M_{\odot}$ NS, whereas the BH fraction shows an approximate monotonic increase
in the 4-25 and 12-25keV energy bands. We calculate the overall ratio of BH to
NS to be $\approx1$ for 4-25 keV and $\approx2$ for 12-25 keV.
