Discovery of a Radio Flare from GRB 990123

We report the discovery of a radio counterpart to GRB 990123. In contrast toprevious well-studied radio afterglows which rise to peak flux on a timescaleof a week and then decay over several weeks to months, the radio emission fromthis GRB was clearly detected one day after the burst, after which it rapidlyfaded away. The simplest interpretation of this ``radio flare'' is that itarises from the reverse shock. In the framework of the afterglow modelsdiscussed to date, a forward shock origin for the flare is ruled out by ourdata. However, at late times, some radio afterglow emission (commensurate withthe observed late-time optical emission, the optical afterglow) is expectedfrom the forward shock. The relative faintness of the observed late-time radioemission provides an independent indication for a jet-like geometry in thisGRB. We use the same radio observations to constrain two key parameters of theforward shock, peak flux and peak frequency, to within a factor of two. Thesevalues are inconsistent with the notion advocated by several authors that theprompt optical emission detected by ROTSE smoothly joins the optical afterglowemission. Finally, with hindsight we now recognize another such radio flare andthis suggests that one out of eight GRBs has a detectable radio flare. Thisabundance coupled with the reverse shock interpretation suggests that the radioflare phenomenon has the potential to shed new light into the physics ofreverse shocks in GRBs.Comment: pages including 2 figures. Accepted by the Astrophys. J. (Letters