THE REDSHIFT DEPENDENCE OF GAMMA-RAY ABSORPTION IN THE ENVIRONMENTS OF STRONG-LINE AGNs

The case of -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 -rayYloud 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 -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 -ray production. Subject headinggs: diffuse radiation — galaxies: active — galaxies: evolution — galaxies: nuclei — gamma rays: theory — radiation mechanisms: nonthermal