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 $\Gamma \lesssim 0.6$) in the 13 deg² NuSTAR serendipitous survey, to search for the most highly obscured active galactic nuclei (AGNs) detected at $>10keV$. 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; $N_H>1.5\times {10}^{24}$ 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 $\approx 5\times {10}^{42}$ 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.