@article{61412ab36c6049dd9ab0cdde645e32cd,
title = "The NuSTAR Serendipitous Survey: Hunting for the Most Extreme Obscured AGN at >10 keV",
abstract = "We identify sources with extremely hard X-ray spectra (i.e., with photon indices of G ≲ 0.6) in the 13 deg2 NuSTAR serendipitous survey, to search for the most highly obscured active galactic nuclei (AGNs) detected at >10 keV. Eight extreme NuSTAR sources are identified, and we use the NuSTAR data in combination with lower-energy X-ray observations (from Chandra, Swift XRT, and XMM-Newton) to characterize the broadband (0.5-24 keV) X-ray spectra. We find that all of the extreme sources are highly obscured AGNs, including three robust Compton-thick (CT; NH > 1.5 1024 cm-2) AGNs at low redshift (z < 0.1) and a likely CT AGN at higher redshift (z = 0.16). Most of the extreme sources would not have been identified as highly obscured based on the low-energy (<10 keV) X-ray coverage alone. The multiwavelength properties (e.g., optical spectra and X-ray-mid-IR luminosity ratios) provide further support for the eight sources being significantly obscured. Correcting for absorption, the intrinsic rest-frame 10-40 keV luminosities of the extreme sources cover a broad range, from 5 ×1042 to 1045 erg s-1. The estimated number counts of CT AGNs in the NuSTAR serendipitous survey are in broad agreement with model expectations based on previous X-ray surveys, except for the lowest redshifts (z < 0.07), where we measure a high CT fraction of fCT obs = 30 % -12 +16 . For the small sample of CT AGNs, we find a high fraction of galaxy major mergers (50% ± 33%) compared to control samples of {"}normal{"} AGNs.",
author = "Lansbury, {G. B.} and Alexander, {D. M.} and J. Aird and P. Gandhi and D. Stern and M. Koss and I. Lamperti and M. Ajello and A. Annuar and Assef, {R. J.} and Ballantyne, {D. R.} and M. Balokovi{\'c} and Bauer, {F. E.} and Brandt, {W. N.} and M. Brightman and Chen, {C. T.J.} and F. Civano and A. Comastri and Moro, {A. Del} and C. Fuentes and Harrison, {F. A.} and S. Marchesi and A. Masini and Mullaney, {J. R.} and C. Ricci and C. Saez and Tomsick, {J. A.} and E. Treister and Walton, {D. J.} and L. Zappacosta",
note = "Funding Information: The authors first thank the anonymous referee for the positive and constructive review. We acknowledge support from a Herchel Smith Postdoctoral Research Fellowship of the University of Cambridge (G.B.L.); the Science and Technology Facilities Council (STFC) grants ST/I001573/1 (D.M.A.) and ST/J003697/2 (P.G.); the ERC Advanced Grant FEEDBACK 340442 at the University of Cambridge (J.A.); the NASA Earth and Space Science Fellowship Program, grant NNX14AQ07H (M.B.); CONICYT-Chile grants FONDECYT Regular 1141218 (F.E.B.), FONDECYT 1120061 and 1160999 (E.T.), and Anillo ACT1101 (F.E.B. and E.T.); the Center of Excellence in Astrophysics and Associated Technologies (PFB 06; F.E.B. and E.T.); the Ministry of Economy, Development, and Tourism{\textquoteright}s Millennium Science Initiative through grant IC120009, awarded to the Millennium Institute of Astrophysics, MAS (F.E.B.); ASI/INAF contract I/037/12/0-011/13 (A.C., A.M., and L.Z.); and Chandra grants GO5-16154X and GO6-17135X (J.A.T.). We thank Yoshihiro Ueda and Roberto Gilli for providing number counts predictions. 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). Publisher Copyright: {\textcopyright} 2017. The American Astronomical Society. All rights reserved.",
year = "2017",
month = sep,
day = "1",
doi = "10.3847/1538-4357/aa8176",
language = "English (US)",
volume = "846",
journal = "Astrophysical Journal",
issn = "0004-637X",
publisher = "IOP Publishing Ltd.",
number = "1",
}