GRB 021004: A Possible Shell Nebula around a Wolf-Rayet Star Gamma-Ray Burst Progenitor

Abstract

The rapid localization of GRB 021004 by the HETE-2 satellite allowed nearly continuous monitoring of its early optical afterglow decay, as well as high-quality optical spectra that determined a redshift of z=2.328 for its host, an active starburst galaxy with strong Lyman-alpha emission and several absorption lines. Spectral observations show multiple absorbers blueshifted by up to 3,155 km/s relative to the host galaxy Lyman-alpha emission.We argue that these correspond to a fragmented shell nebula, gradually enriched by a Wolf-Rayet wind over the lifetime of a massive progenitor bubble. In this scenario, the absorbers can be explained by circumstellar material that have been radiatively accelerated by the GRB emission. Dynamical and photoionization models are used to provide constraints on the radiative acceleration from the early afterglow.