Early optical afterglows from wind-type gamma-ray bursts

Abstract

We study prompt optical emission from reverse shocks in 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 postshock evolution of the fireball ejecta, which determine the decay index of the prompt optical emission and the duration of the radio flare. We discuss distinct emission signatures of the wind environment compared with the constant interstellar medium environment. 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.