On the existence of a reverse shock in magnetized gamma-ray burst ejecta

Free access article

Issue		A&A Volume 478, Number 3, February II 2008


Page(s)		747 - 753
Section		Extragalactic astronomy
DOI		http://dx.doi.org/10.1051/0004-6361:20078931

A&A 478, 747-753 (2008)
DOI: 10.1051/0004-6361:20078931

On the existence of a reverse shock in magnetized gamma-ray burst ejecta

D. Giannios¹, P. Mimica², and M. A. Aloy²

¹ Max Planck Institute for Astrophysics, Box 1317, 85741 Garching, Germany
e-mail: giannios@mpa-garching.mpg.de
² Departamento de Astronomía y Astrofísica, Universidad de Valencia, 46100 Burjassot, Spain

(Received 26 October 2007 / Accepted 23 November 2007)

Abstract
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 and interact with the circumburst medium. Both constant and wind-like density profiles are considered. We show, in contrast to previous estimates, that ejecta with magnetization $\sigma_0\gtrsim$ 1 are not crossed by a reverse shock for a large fraction of the parameter space relevant to GRB flows. Allowing for shell spreading, there is always a relativistic or mildly relativistic reverse shock forming in $\sigma_0\lesssim$ 0.3 ejecta. From this, 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.

Key words: gamma rays: bursts -- magnetohydrodynamics (MHD) -- shock waves

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