Discovery of a Radio Flare from GRB 990123

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  • 30 March 1999
Abstract
We report the discovery of a radio counterpart to GRB~990123. In contrast to previous well-studied radio afterglows which rise to peak flux on a timescale of a week and then decay over several weeks to months, the radio emission from this GRB was clearly detected one day after the burst, after which it rapidly faded away. The simplest interpretation of this ``radio flare'' is that it arises in the reverse shock. Indeed, such radio emission is an inevitable consequence of the prompt bright optical emission seen by ROTSE. A forward shock origin for the flare is definitively ruled out by our data. However, at late times, some radio afterglow emission (commensurate with the observed late-time optical emission, the optical afterglow) is expected from the forward shock. The relative faintness of the observed late-time radio emission provides an independent evidence for a jet-like geometry in this GRB. We use the same radio observations to constrain two key parameters of the forward shock, peak flux and peak frequency, to within a factor of 2. These values are inconsistent with the notion advocated by several authors that the ROTSE optical emission smoothly joins the optical afterglow emission. There is a clear separation in time between the emission from the reverse shock and the afterglow emission. Finally, with hindsight we now recognize another such radio flare and this suggests that one out of eight GRBs has a detectable radio flare. This abundance coupled with the reverse shock interpretation suggests that the radio flare phenomenon can shed new light into the physics of reverse shocks in GRBs.

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