The NuSTAR Extragalactic Survey: A First Sensitive Look at the
  High-Energy Cosmic X-ray Background Population
  We report on the first ten identifications of sources serendipitously
detected by the NuSTAR to provide the first sensitive census of the cosmic
X-ray background (CXB) source population at >10 keV. We find that these
NuSTAR-detected sources are ~100x fainter than those previously detected at >10
keV and have a broad range in redshift and luminosity (z=0.020-2.923 and
L_10-40 keV~4x10^{41}-5x10^{45} erg/s); the median redshift and luminosity are
z~0.7 and L_10-40 keV~3x10^{44} erg/s, respectively. We characterize these
sources on the basis of broad-band ~0.5-32 keV spectroscopy, optical
spectroscopy, and broad-band ultraviolet-to-mid-infrared SED analyzes. We find
that the dominant source population is quasars with L_10-40 keV>10^{44} erg/s,
of which ~50% are obscured with N_H>10^{22} cm^{-2}. However, none of the ten
NuSTAR sources are Compton thick (N_H>10^{24} cm^{-2}) and we place a 90%
confidence upper limit on the fraction of Compton-thick quasars (L_10-40
keV>10^{44} erg/s) selected at >10 keV of ~33% over the redshift range
z=0.5-1.1. We jointly fitted the rest-frame ~10-40 keV data for all of the
non-beamed sources with L_10-40 keV>10^{43} erg/s to constrain the average
strength of reflection; we find R<1.4 for \Gamma=1.8, broadly consistent with
that found for local AGNs observed at >10 keV. We also constrain the host
galaxy masses and find a median stellar mass of ~10^{11} M_sun, a factor ~5
times higher than the median stellar mass of nearby high-energy selected AGNs,
which may be at least partially driven by the order of magnitude higher X-ray
luminosities of the NuSTAR sources. Within the low source-statistic limitations
of our study, our results suggest that the overall properties of the NuSTAR
sources are broadly similar to those of nearby high-energy selected AGNs but
scaled up in luminosity and mass.
