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The Astrophysical Journal






IOP Publishing


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 and 6 × 10−13 erg cm−2 s−1 ( erg cm−2 s−1) and redshifts in the range of . We produce composite spectra from the Chandra + NuSTAR data (, 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 , flatter than the typically found for AGNs. While the spectral slope of BL and X-ray unabsorbed AGNs is consistent with the typical values (), a significant flattening is seen in NL AGNs and heavily absorbed sources ( and , respectively), likely due to the effect of absorption and to the contribution from the Compton reflection component to the high-energy flux ( keV). We find that the typical reflection fraction in our spectra is (for ), 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 decreases with luminosity, with relatively high levels of reflection () for erg s−1 and for erg s−1 AGNs, assuming a fixed spectral slope of .


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