Wind-Interaction Models for the Early Afterglows of Gamma-Ray Bursts: The Case of GRB 021004

Abstract

Wind-interaction models for gamma-ray burst afterglows predict that the optical emission from the reverse shock drops below that from the forward shock within 100s of seconds of the burst. The typical frequency $ν_m$ of the synchrotron emission from the forward shock passes through the optical band typically on a timescale of minutes to hours. Before the passage of $ν_m$, the optical flux evolves as $t^{-1/4}$ and after the passage, the decay steepens to $t^{-(3p-2)/4}$, where $p$ is the exponent for the assumed power-law energy distribution of nonthermal electrons and is typically $\sim 2$. The steepening in the slope of temporal decay should be readily identifiable in the early afterglow light curves. We propose that such a steepening was observed in the R-band light curve of GRB 021004 around day 0.1. Available data at several radio frequencies are consistent with this interpretation, as are the X-ray observations around day~1. The early evolution of GRB 021004 contrasts with that of GRB 990123, which can be described by emission from interaction with a constant density medium.