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 ${\rm{\Gamma }}\lesssim 0.6$) in the 13 deg2 NuSTAR serendipitous survey, to search for the most highly obscured active galactic nuclei (AGNs) detected at $\gt 10\,\mathrm{keV}$. 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; ${N}_{{\rm{H}}}\gt 1.5\times {10}^{24}$ cm−2) AGNs at low redshift ($z\lt 0.1$) 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 ($\lt 10$ 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 ($z\lt 0.07$), where we measure a high CT fraction of ${f}_{\mathrm{CT}}^{\mathrm{obs}}={30}_{-12}^{+16} \%$. 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

The NuSTAR Extragalactic Surveys: X-Ray Spectroscopic Analysis of the Bright Hard-band Selected Sample

We discuss the spectral analysis of a sample of 63 active galactic nuclei (AGN) detected above a limiting flux of S 8 24 keV 7 10 = ´ -14 (– ) erg s cm - - 1 2 in the multi-tiered NuSTAR extragalactic survey program. The sources span a redshift range z = 0 2.1 – (median ázñ = 0.58). The spectral analysis is performed over the broad 0.5–24 keV energy range, combining NuSTAR with Chandra and/or XMM-Newton data and employing empirical and physically motivated models. This constitutes the largest sample of AGN selected at >10 keV to be homogeneously spectrally analyzed at these flux levels. We study the distribution of spectral parameters such as photon index, column density (NH), reflection parameter (R), and 10–40 keV luminosity (LX). Heavily obscured (log cm 2 NH 3 [ ] -2 ) and Compton-thick (CT; log cm 24 NH [ ] -2 ) AGN constitute ∼25% (15–17 sources) and ∼2–3% (1–2 sources) of the sample, respectively. The observed NH distribution agrees fairly well with predictions of cosmic X-ray background population-synthesis models (CXBPSM). We estimate the intrinsic fraction of AGN as a function of NH, accounting for the bias against obscured AGN in a flux-selected sample. The fraction of CT AGN relative to log cm 20 2 NH 4 [ ]– -2 = AGN is poorly constrained, formally in the range 2–56% (90% upper limit of 66%). We derived a fraction ( fabs) of obscured AGN (log cm 22 2 NH 4 [ ]– -2 = ) as a function of LX in agreement with CXBPSM and previous z < 1 X-ray determinations. Furthermore, fabs at z = 0.1 0.5 – and log erg s 43.6 44. Lx 3 ( )– -1 » agrees with observational measurements/trends obtained over larger redshift intervals. We report a significant anti-correlation of R with LX (confirmed by our companion paper on stacked spectra) with considerable scatter around the median R values

Development of targeted viral vectors for cardiovascular gene therapy

No abstract available