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

Abstract

Aims: We determine the cumulative spectral energy distribution (SED) of local active galactic nuclei (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.
Methods: 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.
Results: 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^-1 (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.
Conclusions: 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.