On the contribution of unresolved blazars to the extragalactic gamma ray background

Abstract

We present a model for estimating the blazar contribution to the extragalactic gamma-ray background (EGRB) in the EGRET energy range. It is based on inverse-Compton scattering as the dominant gamma-ray production process in the jets of flat spectrum radio quasars (FSRQs) and BL Lac objects, and on the unification scheme of radio-loud AGN. Depending on the evolutionary behaviour of radio-loud AGN, our estimate for the point source contribution to the extragalactic background emission ranges from about ~20%-40% for a redshift drop off at Z_{max}=3 to around ~35%-85% for a cutoff at Z_{max}=5. Roughly 70%-90% of the AGN contribution to the EGRB would be due to BL~Lacs/FR I objects. However, in case of BL Lacs our model parameters are not entirely in agreement with the results from studies at radio wavelengths indicating systematical uncertainties whereas our estimate for the FSRQ contribution seems to be consistent. Thus we end up with two possibilities depending on whether we underpredict or overpredict the BL Lac contribution: either unresolved AGN can not account for the entire EGRB or unresolved BL Lacs produce the observed background.