Giannios 2007 magnetized reverse shock

主要内容:
Optical flashes may come from the magnetized reverse shock, not the normal reverse shock.

精彩摘抄:


文章信息:
arXiv:0711.1980 [ps, pdf, other]

Title: On the existence of a reverse shock in magnetized GRB ejecta

Authors: D. Giannios, P. Mimica, M. A. Aloy

Comments: 8 pages, 1 figure, A&A submitted

Subjects: Astrophysics (astro-ph)

The role of magnetic fields in gamma-ray burst (GRB) flows remains controversial. The study of the early afterglow phases and, in particular, of the reverse shock dynamics and associated emission offers a promising probe of the magnetization of the ejecta. In this paper, we derive the conditions for the existence of a reverse shock in arbitrarily magnetized ejecta that decelerate interacting with external medium of constant density. We show, in contrast to previous estimates, that ejecta with magnetization larger than unity are not crossed by a reverse shock for a large fraction of the parameter space relevant for GRB flows. Allowing for shell spreading, there is always a relativistic or mildly relativistic reverse shock forming in sigma_o < 0.3 ejecta. Thereby, we conclude that the paucity of optical flashes, believed to be a distinctive signature of a reverse shock, may be explained by the existence of dynamically important magnetic fields in the ejecta.