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

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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 200 and 2× 10⁴ 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.