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

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