2003ApJ...589L..69L

Wind-interaction models for gamma-ray burst (GRB) afterglows predict that the optical emission from the reverse shock drops below that from the forward shock within hundreds of seconds of the burst. The typical frequency ν_mof 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 ∼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.