The NuSTAR Serendipitous Survey: Hunting for the Most Extreme Obscured AGN at >10 keV

Journal
Date Issued
2017
Author(s)
Lansbury, G. B.
•
Alexander, D. M.
•
Aird, J.
•
Gandhi, P.
•
Stern, D.
•
Koss, M.
•
Lamperti, I.
•
Ajello, M.
•
Annuar, A.
•
Assef, R. J.
•
Ballantyne, D. R.
•
Baloković, M.
•
Bauer, F. E.
•
Brandt, W. N.
•
Brightman, M.
•
Chen, C. -T. J.
•
Civano, F.
•
•
Del Moro, A.
•
Fuentes, C.
•
Harrison, F. A.
•
•
Masini, A.
•
Mullaney, J. R.
•
Ricci, C.
•
Saez, C.
•
Tomsick, J. A.
•
Treister, E.
•
Walton, D. J.
•
DOI
Abstract
We identify sources with extremely hard X-ray spectra (I.e., with photon indices of {{Γ }}≲ 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; {N}{{H}}> 1.5× {10}24 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× {10}42 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 {f}{CT}{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.
Volume
846
Issue
1
Start page
20
Uri
Url
Issn Identifier
0004-637X
Ads BibCode
2017ApJ...846...20L
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open.access
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