The Afterglow of GRB 990123 and a Dense Medium

Recent observations show that the temporal decay of the R -band afterglow from GRB 990123 steepened about 2.5 days after the burst. We here propose a possible explanation for such a steepening: a shock expanding in a dense medium has undergone the transition from a relativistic phase to a nonrelativistic phase. We find that this model is consistent with the observations if the medium density is about 3 × 10 6 cm -3 . By fitting our model to the observed optical and X-ray afterglow quantitatively, we further infer the electron and magnetic energy fractions of the shocked medium and find that the two parameters are about 0.1 and 2 × 10 -8 , respectively. The former parameter is near the equipartition value, while the latter is about 6 orders of magnitude smaller than inferred from the GRB 970508 afterglow. We also discuss possibilities that the dense medium can be produced.