A Jet Model for the Afterglow Emission from GRB 000301C

Abstract

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 t${}_{j}et$åisebox-0.5ex 7.3 days, we infer a jet opening angle of þeta${}_0$i̊sebox-0.5ex 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 Ers̊ebox-0.5ex 1.1×10${}^{5}1$ ergs.