Balancing the cosmic energy budget: The cosmic X-ray background, blazars, and the compton thick active galactic nucleus fraction

Abstract

At energies ≳2keV, active galactic nuclei (AGNs) are the source of the cosmic X-ray background (CXB). For AGN population synthesis models to replicate the peak region of the CXB (∼30keV), a highly obscured and therefore nearly invisible class of AGN, known as Compton thick (CT) AGN, must be assumed to contribute nearly a third of the CXB. In order to constrain the CT fraction of AGNs and the CT number density we consider several hard X-ray AGN luminosity functions and the contribution of blazars to the CXB. Following the unified scheme, the radio AGN luminosity function is relativistically beamed to create a radio blazar luminosity function. An average blazar spectral energy density model is created to transform radio luminosity to X-ray luminosity. We find the blazar contribution to the CXB to be 12% in the 0.5-2keV band, 7.4% in the 2-10keV band, 8.9% in the 15-55keV band, and 100% in the MeV region. When blazars are included in CXB synthesis models, CT AGNs are predicted to be roughly one-third of obscured AGNs, in contrast to the prediction of one half if blazars are not considered. Our model implies a BL Lac X-ray duty cycle of ∼13%, consistent with the concept of intermittent jet activity in low power radio galaxies. © 2009. The American Astronomical Society. All rights reserved.