Optical flash of GRB 990123: Constraints on the physical parameters of the reverse shock

The optical flash accompanying GRB 990123 is believed to be powered by the reverse shock of a thin shell. With the best-fit physical parameters for GRB 990123 and the assumption that the parameters in the optical flash are the same as in the afterglow, we show that: 1) the shell is thick rather than thin, and we have provided the light curve for the thick shell case which coincides with the observation; 2) the theoretical peak flux of the optical flash accounts for only 3 x 10(-4) of the observed. In order to remove this discrepancy, the physical parameters, the electron energy and magnetic ratios, epsilon(e) and epsilon(B), should be 0.61 and 0.39, which are very different from their values for the late afterglow.