Balancing the Cosmic Energy Budget: The Cosmic X-ray Background, Blazars and the Compton Thick AGN Fraction

TL;DR

This paper models the contributions of blazars and Compton thick AGN to the cosmic X-ray background, providing new estimates of their fractions and implications for AGN populations.

Contribution

It introduces a model linking radio blazar luminosity to X-ray emission, refining the estimated fraction of Compton thick AGN in the CXB.

Findings

Blazars contribute up to 12% in 0.5-2 keV band.

Including blazars reduces the predicted Compton thick AGN fraction to one-third.

The model suggests a BL Lac X-ray duty cycle of ~13%.

Abstract

At energies ~>2 keV, active galactic nuclei (AGN) are the source of the cosmic X-ray background (CXB). For AGN population synthesis models to replicate the peak region of the CXB (~30 keV), 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 AGN 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 0.5-2 keV band, 7.4% in the 2-10 keV band, 8.9% in the 15-55 keV band, and 100% in the MeV region. When blazars are included in CXB synthesis models, CT AGN are predicted to be roughly one-third of obscured AGN, 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.