The NuSTAR Extragalactic Survey: Average Broadband X-Ray Spectral Properties of the NuSTAR-detected AGNs

Abstract

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 f 3 – 24 keV ≈ 10 − 14 and 6 × 10−13 erg cm−2 s−1 ( f 8 – 24 keV ≈ 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 – 40 keV < 10 44 erg s−1 and R ≈ 0.3 for L 10 – 40 keV > 10 44 erg s−1 AGNs, assuming a fixed spectral slope of Γ = 1.8 .