A Jet Model for the Afterglow Emission from GRB 000301C

We present broadband radio observations of the afterglow of GRB 000301C, spanning from 1.4 to 350 GHz for the period of 3-130 days after the burst. These radio data, in addition to measurements in the optical bands, suggest that the afterglow arises from a collimated outflow, i.e., a jet. To test this 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 occurring 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 than the ISM + jet model. In addition, we present the first clear indication that the reported fluctuations in the optical/IR are achromatic, with similar amplitudes in all bands, and possibly extend into the radio regime. Using the parameters derived from the global fit, in particular a jet break time tjet ≈ 7.3 days, we infer a jet opening angle of θ0 ≈ 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 value, giving E ≈ 1.1 × 1051 ergs.