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Published August 20, 2013 | Submitted + Published
Journal Article Open

The NuSTAR Extragalactic Survey: A First Sensitive Look at the High-energy Cosmic X-Ray Background Population


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 × 10^(41)-5 × 10^(45) 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) > 10^(44) erg s^(–1), of which ≈50% are obscured with N_H ≳ 10^(22) cm^(–2). However, none of the 10 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^(–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) > 10^(43) 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 ≈10^(11) 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.

Additional Information

© 2013 American Astronomical Society. Received 2013 June 2; accepted 2013 June 28; published 2013 August 1. We acknowledge financial support from the Leverhulme Trust (D.M.A. and J.R.M.), the Science and Technology Facilities Council (STFC; D.M.A., A.D.M., and G.B.L.), the SAO grant GO2-13164X (M.A.), NASA Postdoctoral Program at the Jet Propulsion Laboratory (R.J.A.), NSF award AST 1008067 (D.R.B.), Center of Excellence in Astrophysics and Associated Technologies (PFB 06/2007; F.E.B. and E.T.), the Anillo project ACT1101 (F.E.B. and E.T.), FONDECYT Regular 1101024 (F.E.B.), Caltech NuSTAR subcontract 44A-1092750 (W.N.B. and B.L.),NASA ADP grant NNX10AC99G (W.N.B. and B.L.), ASI/INAF grant I/037/12/0 (A.C. and S.P.), CONICYT-Chile under grant FONDECYT 3120198 (C.S.), and FONDECYT regular grant 1120061 (E.T.). We thank the referee for a constructive and positive report. We also thank Michael Koss for the discussion of Swift-BAT results, and Mark Brodwin,Daniel Gettings, John Gizis, Richard Walters, Jingwen Wu, and Dominika Wylezalek for supporting the ground-based follow-up observations. This work was supported under NASA Contract No. NNG08FD60C, and made use of data from the NuSTAR mission, a project led by the California Institute of Technology, managed by the Jet Propulsion Laboratory, and funded by the National Aeronautics and Space Administration. We thank the NuSTAR Operations, Software and Calibration teams for support with the execution and analysis of these observations. This research has made use of the NuSTAR Data Analysis Software (NuSTARDAS) jointly developed by the ASI Science Data Center (ASDC, Italy) and the California Institute of Technology (USA).

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Published - 0004-637X_773_2_125.pdf

Submitted - 1307.1733v1.pdf


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