A jet model for the afterglow emission from GRB 000301C

We present broadband radio observations of the afterglow of GRB 000301C, sp anning from 1.4 to 350 GHz for the period of 3-130 days after the burst. Th ese radio data, in addition to measurements in the optical bands, suggest t hat the afterglow arises from a collimated outflow, i.e., a jet. To test th is hypothesis in a self-consistent manner, we employ a global fit and find that a model of a jet expanding into a constant-density interstellar medium (ISM + jet) provides the best fit to the data. A model of the burst occurr ing in a wind-shaped circumburst medium (wind-only model) can be ruled out, and a wind + jet model provides a much poorer fit of the optical/IR data t han the ISM + jet model. In addition, we present the first clear indication that the reported fluctuations in the optical/IR are achromatic, with simi lar amplitudes in all bands, and possibly extend into the radio regime. Usi ng the parameters derived from the global fit, in particular a jet break ti me t(jet) approximate to 7.3 days, we infer a jet opening angle of theta (0 ) approximate to 0.2 rad; consequently, the estimate of the emitted energy in the GRB itself is reduced by a factor of 50 relative to the isotropic va lue, giving EB approximate to 1.1 x 10(51) ergs.