The redshift dependence of gamma-ray absorption in the environments of strong-line AGNs

The case of gamma-ray absorption due to photon-photon pair production of jet photons in the external photon environments, such as the accretion disk and the broad-line region radiation fields, of gamma-ray-loud active galactic nuclei (AGNs) that exhibit strong emission lines is considered. I demonstrate that this local opacity,'' if detected, will almost unavoidably be redshift-dependent in the sub-TeV range. This introduces nonnegligible biases and complicates approaches for studying the evolution of the extragalactic background light with contemporary GeV instruments such as the Gamma-Ray Large Area Space Telescope ( GLAST), where the gamma-ray horizon is probed by means of statistical analysis of absorption features (e. g., the Fazio-Stecker relation) in AGN spectra at various redshifts. It particularly applies to strong-line quasars, where external photon fields are potentially involved in gamma-ray production.