The NuSTAR extragalactic survey: A first sensitive look at the high-energy cosmic X-ray background population

Abstract

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 × 1041-5 × 1045 erg s-1); the median redshift and luminosity are z 0.7 and L 10-40 keV 3 × 10 44 erg s-1, 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-1, of which 50% are obscured with N H ≳ 1022 cm-2. However, none of the 10 NuSTAR sources are Compton thick (N H ≳ 1024 cm -2) and we place a 90% confidence upper limit on the fraction of Compton-thick quasars (L 10-40 keV > 1044 erg s -1) 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 -1 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. © 2013. The American Astronomical Society. All rights reserved.