The average 0.5-200 kev spectrum of local active galactic nuclei and a new determination of the 2-10 kev luminosity function at z = 0

Abstract

The broad-band X-ray spectra of active galactic nuclei (AGNs) contains information about the nuclear environment from Schwarzschild radii scales (where the primary power law is generated in a corona) to distances of ~1 pc (where the distant reflector may be located). In addition, the average shape of the X-ray spectrum is an important input into X-ray background synthesis models. Here, local (z 0) AGN luminosity functions (LFs) in five energy bands are used as a low-resolution, luminosity-dependent X-ray spectrometer in order to constrain the average AGN X-ray spectrum between 0.5 and 200 keV. The 15-55 keV LF measured by Swift-BAT is assumed to be the best determination of the local LF, and then a spectral model is varied to determine the best fit to the 0.5-2 keV, 2-10 keV, 3-20 keV and 14-195 keV LFs. The spectral model consists of a Gaussian distribution of power laws with a mean photonindex and cutoff energy Ecut, as well as contributions from distant and disc reflection. The reflection strength is parametrized by varying the Fe abundance relative to solar, AFe, and requiring a specific Fe Ka equivalent width (EW). In this way, the presence of the X-ray Baldwin effect can be tested. The spectral model that best fits the four LFs has 1.85 ± 0.15, Ecut = 270+170-80 keV, AFe = 0.3+0.3-0.15. The sub-solar AFe is unlikely to be a true measure of the gas-phase metallicity, but indicates the presence of strong reflection given the assumed Fe Ka EW. Indeed, parametrizing the reflection strength with the R parameter gives R = 1.7+1.7-0.85. There is moderate evidence for no X-ray Baldwin effect. Accretion disc reflection is included in the best-fitting model, but it is relatively weak (broad iron Ka EW < 100 eV) and does not significantly affect any of the conclusions. A critical result of our procedure is that the shape of the local 2-10 keV LFmeasured by HEAO-1 and MAXI is incompatible with the LFs measured in the hard X-rays by Swift-BAT and RXTE. We therefore present a new determination of the local 2-10 keV LF that is consistent with all other energy bands, as well as the de-evolved 2-10 keV LF estimated from the XMM-Newton Hard Bright Survey. This new LF should be used to revise current measurements of the evolving AGN LF in the 2-10 keV band. Finally, the suggested absence of the X-ray Baldwin effect points to a possible origin for the distant reflector in dusty gas not associated with the AGN obscuring medium. This may be the same material that produces the compact 12 μm source in local AGNs. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.