2018ApJ...854...33Z

We discuss the spectral analysis of a sample of 63 active galactic nuclei (AGN) detected above a limiting flux of S(8–24keV)=7×10^–14ergs^–1cm^–2 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 > 10keV to be homogeneously spectrally analyzed at these flux levels. We study the distribution of spectral parameters such as photon index, column density (N_H), reflection parameter (R), and 10-40 keV luminosity (L_X). Heavily obscured (log[N_H/cm^–2]≥ 23) and Compton-thick (CT; log[N_H/cm^–2]≥ 24) AGN constitute ∼25% (15-17 sources) and ∼2-3% (1-2 sources) of the sample, respectively. The observed N_H 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 N_H, accounting for the bias against obscured AGN in a flux-selected sample. The fraction of CT AGN relative to log[N_H/cm^–2]=20–24 AGN is poorly constrained, formally in the range 2-56% (90% upper limit of 66%). We derived a fraction (f_abs) of obscured AGN (log[N_H/cm^–2]=22–24) as a function of L_X in agreement with CXBPSM and previous z< 1 X-ray determinations. Furthermore, f_abs at z=0.1–0.5 and log(L_x/ergs^–1)~43.6–44.3 agrees with observational measurements/trends obtained over larger redshift intervals. We report a significant anti-correlation of R with L_X (confirmed by our companion paper on stacked spectra) with considerable scatter around the median R values.