Nanjing Univ, Dept Astron, Nanjing 210093, Peoples R China
; Peking Univ, Joint Beijing Astrophys, Chinese Acad Sci, Beijing 100871, Peoples R China
; China Ctr Adv Sci & Technol, World Lab, Beijing 100080, Peoples R China
; Chinese Acad Sci, Inst High Energy Phys, Lab Cosm Ray & High Energy Astrophys, Beijing, Peoples R China
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 x 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 x 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.