The Enigmatic Radio Afterglow of GRB 991216

We present broadband radio observations spanning 1.4-350 GHz of the afterglow of GRB 991216, taken 1-80 days after the burst. The optical and X-ray afterglow of this burst were fairly typical and are explained by a jet fireball. In contrast, the radio afterglow is unusual in two respects: (1) the radio light curve does not show the usual rise to maximum flux on timescales of weeks and instead appears to be declining already on day 1; and (2) the power-law indices show significant steepening from the radio through the X-ray bands. We show that the standard fireball model, in which the afterglow is from a forward shock, is unable to account for point 1, and we conclude that the bulk of the radio emission must arise from a different source. We consider two models, neither of which can be ruled out with the existing data. In the first (conventional) model, the early radio emission is attributed to emission from the reverse shock, as in the case of GRB 990123. In the second "dual fireball" model, the radio emission originates from the forward shock of an isotropically energetic fireball (1054 ergs) expanding into a tenuous medium (10-4 cm-3), while the optical and X-ray emission originate in a jetlike outflow. Finally, we note that the near-IR bump of the afterglow is similar to that seen in GRB 971214, and no fireball model can explain this bump.