THE POLARIZATION DEPENDENCE OF gamma gamma ABSORPTION-IMPLICATIONS FOR gamma -RAY BURSTS AND BLAZARS

• Published in: The Astrophysical journal Vol. 795; no. 1; pp. 1 - 4
• Main Author: Bottcher, M
• Format: Journal Article
• Language: English
• Published: 01.01.2014
• Subjects: Blazars; Breaking; Bursts; Magnetic fields; Opacity; Photons; Polarization; Spectra
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1088/0004-637X/795/1/35

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 overestimation 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; 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. This amount is distinctly different from the change in polarization expected if the same spectral break were 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.