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

TL;DR

This paper explores how gamma-ray absorption in strong-line AGN environments varies with redshift, affecting the interpretation of gamma-ray spectra and the study of extragalactic background light evolution.

Contribution

It demonstrates the redshift dependence of local gamma-ray opacity in strong-line AGN, highlighting its impact on gamma-ray absorption studies and background light evolution.

Findings

Gamma-ray absorption due to photon-photon pair production is redshift-dependent.

This dependence introduces biases in extragalactic background light measurements.

External photon fields influence gamma-ray production in strong-line quasars.

Abstract

The case of gamma-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 gamma-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 gamma-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 gamma-ray production.