GRB 990123 revisited: Further Evidence for a Reverse Shock

Recently we have presented a new theoretical analysis of the reverse shock emission. We use this analysis here to revisit GRB 990123. We find new and compelling evidences that the optical flash and the radio flare of GRB 990123 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 ISM and that in this burst the reverse shock emission dominated 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.