The NuSTAR Serendipitous Survey: Hunting for the Most Extreme Obscured AGN at >10 keV
Harrison, F. A.; Stern, D.; Alexander, D. M.; Treister, E.; Bauer, F. E.; Brandt, W. N.; Comastri, A.; Ajello, M.; Gandhi, P.; Tomsick, J. A.; Civano, F.; Aird, J.; Marchesi, S.; Brightman, M.; Chen, C. -T. J.; Del Moro, A.; Lansbury, G. B.; Assef, R. J.; Ballantyne, D. R.; Mullaney, J. R.; Saez, C.; Ricci, C.; Koss, M.; Baloković, M.; Masini, A.; Walton, D. J.; Zappacosta, L.; Annuar, A.; Lamperti, I.; Fuentes, C.
United Kingdom, United States, Switzerland, Chile, Italy, Germany, China
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.