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

The case of γ-ray absorption due to photon-photon pair production of jet photons in the external photon environment like accretion disk and broad-line region radiation field of γ-ray loud active galactic nuclei (AGN) 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 non-negligible biases, and complicates approaches for studying the evolution of the extragalactic background light with contemporary GeV instruments like e.g. the Gamma-ray Large Area Space Telescope (GLAST), etc., where the γ-ray horizon is probed by means of statistical analysis of absorption features (e.g. Fazio-Stecker relation, etc.) in AGN spectra at various redshifts. It particularly applies to strong-line quasars where external photon fields are potentially involved in γ-ray production. Subject headings: gamma rays: theory – diffuse radiation – radiation mechanisms: non-thermal – galaxies: active, nuclei – galaxies: evolution