The NuSTAR Extragalactic Survey: Average Broadband X-Ray Spectral Properties of the NuSTAR-detected AGNs
A. Del Moro ; D. M. Alexander ; J. A. Aird ; [...] S. Puccetti ; et. al.
Oct - 2017
DOI: 10.3847/1538-4357/aa9115

journal : The Astrophysical Journal

Volume : 849 ; Issue : 1
type: Article Journal

We present a study of the average X-ray spectral properties of the sources detected by the NuSTAR extragalactic survey, comprising observations of the E-CDFS, EGS and COSMOS fields. The sample includes 182 NuSTAR sources (64 detected at 8-24 keV), with 3-24 keV fluxes ranging between f3−24keV≈10−14 and 6×10−13 erg/cm2/s (f8−24keV≈3×10−14−3×10−13 erg/cm2/s) and redshifts 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) AGN, 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 Γ≈1.8 typically found for AGN. While the spectral slope of BL and X-ray unabsorbed AGN is consistent with typical values (Γ=1.79+0.01−0.01), a significant flattening is seen in NL AGN 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 Compton reflection 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 column density. While there is no significant evidence for a dependence of the photon index with X-ray luminosity in our sample, we find that R decreases with luminosity, with relatively high levels of reflection (R≈1.2) for L10−40keV<1044 erg/s and R≈0.3 for L10−40keV>1044 erg/s AGN, assuming Γ=1.8.

keywords : galaxies: active - quasars: general - X-rays: galaxies - surveys