The nustar extragalactic survey: average broadband X - ray spectral properties of the NuSTAR - detected AGNs

Abstract

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 f3(-24keV) approximate to 10(-14) and 6 x 10(-13) erg cm(-2) S-1 (f(8-24 keV) approximate to 3 x 10(-14)-3 x 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 approximate to 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 Gamma = 1.651(-0.03)(+0.03), flatter than the Gamma approximate to 1.8 typically found for AGNs. While the spectral slope of BL and X-ray unabsorbed AGNs is consistent with the typical values (Gamma = 1.79(-0.01)(+0.01)), a significant flattening is seen in NL AGNs and heavily absorbed sources (Gamma = 1.60(-0.05)(+0.08) and Gamma = 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 approximate to 0.5 (for F = 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 approximate to 1.2) for L10-40 keV < 10(44) erg s(-1) and R approximate to 0.3 for L10-40 (keV) > 10(44) erg s(-1) AGNs, assuming a fixed spectral slope of Gamma = 1.8.