GRB 990123: The optical flash and the fireball model

We show that the ongoing observations of the remarkable burst GRB 990123 agree with the predictions of the fireball model. Specifically, the observations confirm the recent prediction that the reverse shock propagating into the ejecta would produce a very strong prompt optical flash. This reverse shock has also produced the 8.46 GHz radio signal, observed after 1 day. The forward shock, which is the origin of the classical afterglow, has produced the prompt X-ray signal as well as the late optical and IR emission. The observations suggest that the initial Lorentz factor of the ejecta was similar to 200. Within factors of order unity, this crude model explains all current observations of GRB 990123.