A Possible Explanation for the Radio ``Flare'' in the Early Afterglow of GRB990123

Abstract

We suggest that the deceleration of the relativistic shock by a denser part of the interstellar medium off line-of-sight produced the observed radio ``flare'' in the early afterglow of GRB990123. We find that this scenario is consistent with observations if the particle number density of this denser part of the medium is between $\sim 200$ and $\sim 2\times 10^4$ cm$^{-3}$. Because of the premature deceleration of part of the shock, the later stage of the afterglow should decay modestly faster than the powerlaw expected from an isotropic shock propagation.

14 pages including 3 figures, submitted to ApJL