2005ApJ...619L.147N

Recently, we presented a new theoretical analysis of the reverse shock emission. We use this analysis here to revisit the early afterglow of GRB 990123. The first mechanism that was suggested as the source of the optical flash observed in GRB 990123 was the reverse shock. However, it was shown later that other processes can generate this flash as well. We find new and compelling evidence that the optical flash and the following radio flare of GRB 990123 did resulted from a reverse shock. This suggests that a significant fraction of the energy of the relativistic ejecta must have been carried by baryons. It also suggests that the external medium is an interstellar medium and that in this burst the reverse shock emission dominates at early time over other possible processes. We use the early optical emission to constrain the physical parameters of the original ejecta and the microscopic parameters in the emitting reverse-shocked region.