Alexander, D. M.3; Stern, D.19; Del Moro, A.3; Lansbury, G. B.3; Assef, R. J.19; Aird, J.3; Ajello, M.5; Ballantyne, D. R.20; Bauer, F. E.7; Boggs, S. E.5; Brandt, W. N.21; Christensen, Finn Erland1; Civano, F.22; Comastri, A.10; Craig, W. W.11; Elvis, M.12; Grefenstette, B. W.19; Hailey, C. J.23; Harrison, F. A.19; Hickox, R. C.22; Luo, B.21; Madsen, K. K.19; Mullaney, J. R.3; Perri, M.14; Puccetti, S.14; Saez, C.7; Treister, E.15; Urry, C. M.16; Zhang, W. W.24; Bridge, C. R.19; Eisenhardt, P. R. M.19; Gonzalez, A. H.18; Miller, S. H.19; Tsai, C. W.19
1 National Space Institute, Technical University of Denmark2 Astrophysics, National Space Institute, Technical University of Denmark3 Durham University4 California Institute of Technology5 University of California at Berkeley6 Georgia Institute of Technology7 Pontifícia Universidade Católica8 Pennsylvania State University9 Dartmouth College10 National Institute for Astrophysics11 Technical University of Denmark12 Harvard-Smithsonian Center for Astrophysics13 Columbia University14 ASI Science Data Center15 Universidad de Concepcion16 Yale University17 NASA Goddard Space Flight Center18 University of Florida19 California Institute of Technology20 Georgia Institute of Technology21 Pennsylvania State University22 Dartmouth College23 Columbia University24 NASA Goddard Space Flight Center
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 L10-40 keV ≈ 4 × 1041 -5 × 1045 erg s-1); the median redshift and luminosity are z ≈ 0.7 and L 10-40 keV ≈ 3 × 1044 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 L10-40 keV > 1044 erg s-1, of which ≈ 50% are obscured with NH ≳ 1022 cm-2. However, none of the 10 NuSTAR sources are Compton thick (NH ≳ 1024 cm-2) and we place a 90% confidence upper limit on the fraction of Compton-thick quasars (L10-40 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 L10-40 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.