Issue |
A&A
Volume 494, Number 3, February II 2009
|
|
---|---|---|
Page(s) | 879 - 890 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361:200810756 | |
Published online | 11 December 2008 |
Deceleration of arbitrarily magnetized GRB ejecta: the complete evolution
1
Departamento de Astronomía y Astrofísica, Universidad de Valencia, 46100 Burjassot, Spain e-mail: Petar.Mimica@uv.es
2
Max Planck Institute for Astrophysics, Box 1317, 85741 Garching, Germany
Received:
6
August
2008
Accepted:
4
November
2008
Context. The role of magnetic fields in gamma-ray burst (GRB) flows remains debated. If of sufficient strength, they can leave their signature on the initial phases of the afterglow by substantially changing the backreaction of the flow as a consequence of its interaction with the external medium.
Aims. We attempt to understand quantitatively the dynamical effect and observational signatures of GRB ejecta magnetization on the onset of the afterglow.
Methods. We perform ultrahigh-resolution, one-dimensional, relativistic MHD simulations of the interaction between a radially expanding, magnetized ejecta with the interstellar medium. We require ultrahigh numerical resolution because of the extreme jump conditions in the region of interaction between the ejecta and the circumburst medium. We study the complete evolution of an ultrarelativistic shell to the self-similar asymptotic phase.
Results. Our simulations demonstrate that the complete evolution can be
characterized in terms of two parameters, the ξ parameter
introduced by Sari and Piran and the magnetization . We
use this fact in producing numerical models in which the shell
Lorentz factor
is between 10 and 20 and rescaling the
results to arbitrarily large values of
. We find that the
reverse shock is typically weak or absent for ejecta characterized
by
. The onset of the forward shock emission is
strongly dependent on the magnetization. On the other hand, the
magnetic energy of the shell is transferred into the external medium
on a short timescale (of several times the duration of the
burst). The later forward shock emission contains no information
about the initial magnetization of the flow. The asymptotic
evolution of strongly magnetized shells, after experiencing
significant deceleration, resembles that of hydrodynamic shells,
i.e. they enter fully into the Blandford-McKee self-similar regime.
Key words: gamma rays: bursts / methods: numerical / magnetohydrodynamics (MHD) / shock waves
© ESO, 2009
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