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

We identify sources with extremely hard X-ray spectra (i.e., with photon indices of Γ ≾ 0.6) in the 13 deg^2 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 NuSTARdata 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 x 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 x 10^(42) to 10^(45) erg s^(−1). The estimated number counts of CT AGNs in the NuSTARserendipitous 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^(obs)_(CT) = 30^(+16)_(-12)%. 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.