2017ApJ...849...57D

We present a study of the average X-ray spectral properties of the sources detected by the NuSTAR extragalactic survey, comprising observations of the Extended Chandra Deep Field South (E-CDFS), Extended Groth Strip (EGS), and the Cosmic Evolution Survey (COSMOS). The sample includes 182 NuSTAR sources (64 detected at 8-24 keV), with 3-24 keV fluxes ranging between f_3–24keV~10^–14 and 6 x 10^–13 erg cm^–2 s^–1 (f_8–24keV~3×10^–14–3×10^–13 erg cm^–2 s^–1) and redshifts in the range of z=0.04–3.21. We produce composite spectra from the Chandra + NuSTAR data (E~2–40 keV, rest frame) for all the sources with redshift identifications (95%) and investigate the intrinsic, average spectra of the sources, divided into broad-line (BL) and narrow-line (NL) active galactic nuclei (AGNs), and also in different bins of X-ray column density and luminosity. The average power-law photon index for the whole sample is Γ=1.65_–0.03^+0.03, flatter than the Γ~1.8 typically found for AGNs. While the spectral slope of BL and X-ray unabsorbed AGNs is consistent with the typical values (Γ=1.79_–0.01^+0.01), a significant flattening is seen in NL AGNs and heavily absorbed sources (Γ=1.60_–0.05^+0.08 and Γ=1.38_–0.12^+0.12, respectively), likely due to the effect of absorption and to the contribution from the Compton reflection component to the high-energy flux (E 10 keV). We find that the typical reflection fraction in our spectra is R~0.5 (for Γ=1.8), with a tentative indication of an increase of the reflection strength with X-ray column density. While there is no significant evidence for a dependence of the photon index on X-ray luminosity in our sample, we find that R decreases with luminosity, with relatively high levels of reflection (R~1.2) for L_10–40keV 10⁴⁴ erg s^–1 and R~0.3 for L_10–40keV 10⁴⁴ erg s^–1 AGNs, assuming a fixed spectral slope of Γ=1.8.