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 NuSTARextragalactic 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 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 ≈ 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^(+0.08)_(-0.05) 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.