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/cm2 in the multi-tiered NuSTAR Extragalactic Survey program. The sources span a redshift range z=0-2.1 (median ⟨z⟩=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[NH/cm−2]≥23) and Compton Thick (CT; log[NH/cm−2]≥24) AGN constitute ∼25% (15-17 sources) and ∼2-3% ( 1-2 sources) of the sample, respectively. The observed NH distribution fairly agrees 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[NH/cm−2]=20−24 AGN is poorly constrainted, formally in the range 2-56% (90% upper limit of 66%). We derived a fraction (fabs) of obscured AGN (log[NH/cm−2]=22−24) 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(LX/erg s−1)≈43.6−44.3 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.