  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 from the reverse shock. In the framework of the afterglow models
discussed to date, a forward shock origin for the flare is 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 indication 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 two. These
values are inconsistent with the notion advocated by several authors that the
prompt optical emission detected by ROTSE smoothly joins the optical 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 has the potential to shed new light into the physics of
reverse shocks in GRBs.
