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 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}<SUB>3{--</SUB>24{keV}}≈ {10}<SUP>-14</SUP> and 6 × 10<SUP>-13</SUP> erg cm<SUP>-2</SUP> s<SUP>-1</SUP> ({f}<SUB>8{--</SUB>24{keV}}≈ 3× {10}<SUP>-14</SUP>{--}3× {10}<SUP>-13</SUP> erg cm<SUP>-2</SUP> s<SUP>-1</SUP>) 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}<SUB>-0.03</SUB><SUP>+0.03</SUP>, 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}<SUB>-0.01</SUB><SUP>+0.01</SUP>), a significant flattening is seen in NL AGNs and heavily absorbed sources ({{Γ }}={1.60}<SUB>-0.05</SUB><SUP>+0.08</SUP> and {{Γ }}={1.38}<SUB>-0.12</SUB><SUP>+0.12</SUP>, 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}<SUB>10{--</SUB>40{keV}}< {10}<SUP>44</SUP> erg s<SUP>-1</SUP> and R≈ 0.3 for {L}<SUB>10{--</SUB>40{keV}}> {10}<SUP>44</SUP> erg s<SUP>-1</SUP> AGNs, assuming a fixed spectral slope of {{Γ }}=1.8.