The NuSTAR Extragalactic Survey: Average broad-band X-ray spectral properties of the NuSTAR detected AGN

TL;DR

This study analyzes the average X-ray spectral properties of 182 NuSTAR-detected AGN across multiple fields, revealing flatter spectra than typical and a reflection component that varies with luminosity and absorption.

Contribution

It provides the first comprehensive average broad-band X-ray spectral analysis of NuSTAR extragalactic sources, including distinctions between AGN types and absorption levels.

Findings

Average photon index is 1.65, flatter than typical AGN spectra.

Reflection fraction is about 0.5, increasing with absorption.

Reflection strength decreases with X-ray luminosity.

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 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 $f_{\rm 3-24 keV}\approx10^{-14}$ and $6\times10^{-13}$ erg/cm$^2$/s ($f_{\rm 8-24 keV}\approx3\times10^{-14}-3\times10^{-13}$ erg/cm$^2$/s) and redshifts of $z=0.04-3.21$. We produce composite spectra from the Chandra+NuSTAR data ($E\approx2-40$ 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 $\Gamma=1.65_{-0.03}^{+0.03}$, flatter than $\Gamma\approx1.8$ typically found for AGN. While the spectral slope of BL and X-ray unabsorbed AGN is consistent with typical values ($\Gamma=1.79_{-0.01}^{+0.01}$), a significant flattening is seen in NL AGN and heavily-absorbed sources ($\Gamma=1.60_{-0.05}^{+0.08}$ and $\Gamma=1.38_{-0.12}^{+0.12}$, 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 $R\approx0.5$ (for $\Gamma=1.8$), 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 ($R\approx1.2$) for $L_{\rm 10-40 keV}<10^{44}$ erg/s and $R\approx0.3$ for $L_{\rm 10-40 keV}>10^{44}$ erg/s AGN, assuming $\Gamma=1.8$.