Lansbury, G. B.G. B.LansburyAlexander, D. M.D. M.AlexanderAird, J.J.AirdGandhi, P.P.GandhiStern, D.D.SternKoss, M.M.KossLamperti, I.I.LampertiAjello, M.M.AjelloAnnuar, A.A.AnnuarAssef, R. J.R. J.AssefBallantyne, D. R.D. R.BallantyneBaloković, M.M.BalokovićBauer, F. E.F. E.BauerBrandt, W. N.W. N.BrandtBrightman, M.M.BrightmanChen, C. -T. J.C. -T. J.ChenCivano, F.F.CivanoCOMASTRI, AndreaAndreaCOMASTRIDel Moro, A.A.Del MoroFuentes, C.C.FuentesHarrison, F. A.F. A.HarrisonMARCHESI, STEFANOSTEFANOMARCHESIMasini, A.A.MasiniMullaney, J. R.J. R.MullaneyRicci, C.C.RicciSaez, C.C.SaezTomsick, J. A.J. A.TomsickTreister, E.E.TreisterWalton, D. J.D. J.WaltonZappacosta, L.L.Zappacosta2020-08-272020-08-2720170004-637Xhttp://hdl.handle.net/20.500.12386/26877We identify sources with extremely hard X-ray spectra (I.e., with photon indices of {{Γ }}≲ 0.6) in the 13 deg<SUP>2</SUP> 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; {N}<SUB>{{H</SUB>}}> 1.5× {10}<SUP>24</SUP> cm<SUP>-2</SUP>) 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× {10}<SUP>42</SUP> to 10<SUP>45</SUP> erg s<SUP>-1</SUP>. 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 {f}<SUB>{CT</SUB>}<SUP>{obs</SUP>}={30}<SUB>-12</SUB><SUP>+16</SUP> % . 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.STAMPAenArticle10.3847/1538-4357/aa81762-s2.0-85029043949000408362800020https://iopscience.iop.org/article/10.3847/1538-4357/aa81762017ApJ...846...20LFIS/05 - ASTRONOMIA E ASTROFISICAERC sectors::Physical Sciences and Engineering::PE9 Universe sciences: astro-physics/chemistry/biology; solar systems; stellar, galactic and extragalactic astronomy, planetary systems, cosmology, space science, instrumentation