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

• Published in: arXiv.org
• Main Author: Boettcher, Markus
• Format: Paper Journal Article
• Language: English
• Published: Ithaca Cornell University Library, arXiv.org 05.09.2014
• Subjects: Absorption cross sections; Blazars; Dependence; Diagnostic systems; Electron distribution; Emission; Gamma ray bursts; Gamma rays; Magnetic fields; Opacity; Photons; Physics - High Energy Astrophysical Phenomena; Polarization
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.1409.1674

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.