The Polarization Dependence of Gamma-Gamma Absorption - Implications for Gamma-Ray Bursts and Blazars

This paper presents an analysis of the dependence of the opacity for high-energy gamma-rays to gamma-gamma absorption by low-energy photons, on the polarization of the gamma-ray and target photons. This process has so far only been considered using the polarization-averaged gamma-gamma absorption cross section. It is demonstrated that in the case of polarized gamma-ray emission, subject to source-intrinsic gamma-gamma absorption by polarized target photons, this may lead to a slight over-estimation of the gamma-gamma opacity by up to ~ 10 % in the case of a perfectly ordered magnetic field. Thus, for realistic astrophysical scenarios with partially ordered magnetic fields, the use of the polarization-averaged gamma-gamma cross section is justified for practical purposes, such as estimates of minimum Doppler factors inferred for gamma-ray bursts and blazars, based on gamma-gamma transparency arguments, and this paper quantifies the small error incurred by the unpolarized-radiation approximation. Furthermore, it is shown that polarization-dependent gamma-gamma absorption of initially polarized gamma-rays can lead to a slight increase in the polarization beyond the spectral break caused by gamma-gamma absorption, to an amount distinctly different from the change in polarization expected if the same spectral break was produced by a break in the underlying electron distribution. This may serve as a diagnostic of whether gamma-gamma absorption is relevant in sources such as gamma-ray bursts and blazars where the gamma-ray emission may be intrinsically highly polarized.