The NuSTAR Extragalactic Surveys: X-ray spectroscopic analysis of the bright hard-band selected sample

Abstract

We discuss the spectral analysis of a sample of 63 Active Galactic Nuclei (AGN) detected above a limiting flux of $S(8-24$ keV$)=7\times 10^{-14}$ erg/s/cm${}^2$ 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 $>10$ keV to be homogeneously spectrally analyzed at these flux levels. We study the distribution of spectral parameters such as photon index, column density ($N_{\mathrm{H}}$), reflection parameter ($R$) and 10-40 keV luminosity ($L_X$). Heavily obscured ($\log [N_{\mathrm{H}}/c{m}^{-2}]\ge 23$) and Compton Thick (CT; $\log [N_{\mathrm{H}}/c{m}^{-2}]\ge 24$) AGN constitute $\sim$25% (15-17 sources) and $\sim$2-3% ( 1-2 sources) of the sample, respectively. The observed $N_{\mathrm{H}}$ 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 $N_H$, accounting for the bias against obscured AGN in a flux-selected sample. The fraction of CT AGN relative to $\log [N_{\mathrm{H}}/c{m}^{-2}]=20-24$ AGN is poorly constrainted, formally in the range 2-56% (90% upper limit of 66%). We derived a fraction ($f_{abs}$) of obscured AGN ($\log [N_{\mathrm{H}}/c{m}^{-2}]=22-24$) 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 $\log (L_X/erg$ $s^{-1})\approx 43.6-44.3$ 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.

Comments: 38 pages, 17 figures, 8 tables. Accepted for publication in ApJ