2013ApJ...773..125A

We report on the first 10 identifications of sources serendipitously detected by the Nuclear Spectroscopic Telescope Array (NuSTAR) to provide the first sensitive census of the cosmic X-ray background source population at ≳ 10 keV. We find that these NuSTAR-detected sources are ~100 times 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_~ 4 x10⁴¹-5x10⁴⁵ erg/s); the median redshift and luminosity are z ~ 0.7 and L_10-40 keV_~ 3x10⁴⁴ 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 spectral energy distribution analyses. We find that the dominant source population is quasars with L_10-40 keV_> 10⁴⁴ erg/s, of which ~50% are obscured with N_H ≳ 10²²/cm2. However, none of the 10 NuSTAR sources are Compton thick (N_H ≳ 10²⁴/cm2) and we place a 90% confidence upper limit on the fraction of Compton-thick quasars (L_10-40 keV_> 10⁴⁴ 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_> 10⁴³ erg/s to constrain the average strength of reflection; we find R < 1.4 for Γ = 1.8, broadly consistent with that found for local active galactic nuclei (AGNs) observed at ≳ 10 keV. We also constrain the host-galaxy masses and find a median stellar mass of ~10¹¹ M_☉, 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.