Optical flashes and very early afterglows in wind environments

Abstract

The interaction of a relativistic fireball with its ambient medium is described through two shocks: a reverse shock that propagates into the fireball, and a forward shock that propagates into the medium. The observed optical flash of GRB 990123 has been considered to be the emission from such a reverse shock. The observational properties of afterglows suggest that the progenitors of some γ-ray bursts (GRBs) may be massive stars and their surrounding media may be stellar winds. We here study very early afterglows from the reverse and forward shocks in winds. An optical flash mainly arises from the relativistic reverse shock, while a radio flare is produced by the forward shock. The peak flux densities of optical flashes are larger than 1 Jy for typical parameters, if we do not take into account some appropriate dust obscuration along the line of sight. The radio flare always has a long-lasting constant flux, which will not be covered up by interstellar scintillation. The non-detections of optical flashes brighter than about ninth magnitude may constrain the GRB isotropic energies to be no more than a few 1052 erg and wind intensities to be relatively weak.