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

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 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,-3.21 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 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.