Cumulative hard X-ray spectrum of local AGN: a link to the cosmic X-ray background

Abstract

We determine the cumulative spectral energy distribution (SED) of local AGN in the 3-300 keV band and compare it with the spectrum of the cosmic X-ray background (CXB) in order to test the widely accepted paradigm that the CXB is a superposition of AGN and to place constraints on AGN evolution. We performed a stacking analysis of the hard X-ray spectra of AGN detected in two recent all-sky surveys, performed by the IBIS/ISGRI instrument aboard INTEGRAL and by the PCA instrument aboard RXTE, taking into account the space densities of AGN with different luminosities and absorption column densities. We derived the collective SED of local AGN in the 3-300 keV energy band. Those AGN with luminosities below 10^43.5 erg/s (17-60 keV) provide the main contribution to the local volume hard X-ray emissivity, at least 5 times more than more luminous objects. The cumulative spectrum exhibits (although with marginal significance) a cutoff at energies above 100-200 keV and is consistent with the CXB spectrum if AGN evolve over cosmic time in such a way that the SED of their collective high-energy emission has a constant shape and the relative fraction of obscured AGN remains nearly constant, while the AGN luminosity density undergoes strong evolution between z~1 and z=0, a scenario broadly consistent with results from recent deep X-ray surveys. The first direct comparison between the collective hard X-ray SED of local AGN and the CXB spectrum demonstrates that the popular concept of the CXB being a superposition of AGN is generally correct. By repeating this test using improved AGN statistics from current and future hard X-ray surveys, it should be possible to tighten the constraints on the cosmic history of black hole growth.

12 pages, 9 figures. Revised version accepted for publication in A&A

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• LeMMINGs IV: The X-ray properties of a statistically-complete sample of the nuclei in active and inactive galaxies from the Palomar sample
• INTEGRAL view of AGN
• Large-scale variations of the cosmic X-ray background and the X-ray emissivity of the local Universe
• A comprehensive X-ray and multiwavelength study of the Colliding Galaxy Pair NGC2207/IC2163
• Decrease in the Brightness of the Cosmic X-ray and Soft Gamma-ray Background toward Clusters of Galaxies
• Measurements of the Cosmic X-ray Background of the Universe and the MVN Experiment
• SRG/ART-XC all-sky X-ray survey: Catalog of sources detected during the first year