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We use data on the image size of the radio afterglow of GRB 030329 (Taylor etal. 2004) to constrain the physical parameters of this explosion. Together withthe observed broad band spectrum, this data over-constrains the physicalparameters, thus enabling to test different GRB jet models for consistency. Weconsider two extreme models for the lateral spreading of the jet: model 1 withrelativistic expansion in the local rest frame, and model 2 with little lateralexpansion as long as the jet is highly relativistic. We find that both modelsare consistent with the data for a uniform external medium, while for a stellarwind environment model 1 is consistent with the data but model 2 is disfavoredby the data. Our derivations can be used to place tighter constraints on thedynamics and structure of GRB jets in future afterglows, following a densermonitoring campaign for the temporal evolution of their image size.Comment: 12 pages, 6 figues; submitted to Ap

Implications of the Measured Image Size for the Radio Afterglow of GRB 030329