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Early Optical Afterglows from Wind-Type Gamma-Ray Bursts
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Early Optical Afterglows from Wind-Type Gamma-Ray Bursts
Shiho Kobayashi1,2 and Bing Zhang1
1Department of Astronomy Astrophysics, Pennsylvania State University, University
Park, PA 16802
2Center for Gravitational Wave Physics, Pennsylvania State University, University Park,
PA 16802
ABSTRACT
We study prompt optical emission from reverse shocks in the wind-type
gamma-ray bursts. The emission is evaluated in both the thick and thin shell
regimes. We discuss the angular time delay effect and the post-shock evolution of
the fireball ejecta, which determine the decay index of the prompt optical emis-
sion and the duration of the radio flare. We discuss distinct emission signatures
of the wind environment compared with the constant interstellar medium envi-
ronment. We also present two recipes for directly constraining the initial Lorentz
factor of the fireball using the reverse and forward shock optical afterglow data
for the wind case.
Subject headings: gamma rays: bursts — hydrodynamics — relativity — shock
waves
1. Introduction
The gamma-ray burst (GRB) afterglow observations are usually explained by a model
in which a relativistic fireball shell (ejecta) is expanding into a uniform interstellar medium
(ISM). However, there is observational evidence suggesting a link between GRBs and massive
stars or star formation (e.g. Me?sza?ros 2001 for a review). An important consequence of a
massive star origin for the afterglow is that the fireball shell should be expanding into a
pre-burst stellar wind of the progenitor star with a density distribution of ρ ∝ R?2 (e.g.
Chevalier Li 1999; Me?sza?ros, Rees Wijers 1998; Dai Lu 1998). This wind model is
discussed to be consistent with some GRB afterglow data (Chevalier Li 1999, 2000).
It is expected that many early optical afterglows will be discovered soon in the obser-
vational campaigns led by HETE-2 and Swift. Long time span (from right after the GRB
–
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