The NuSTAR serendipitous survey: hunting for the most extreme obscured AGN at > 10keV

Abstract

We identify sources with extremely hard X-ray spectra (i.e., with photon indices of Gamma less than or similar to 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 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 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 approximate to 5 X 10(42) to 10(45) 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.