A Jet Model for the Afterglow Emission from GRB 000301C

We present broad-band radio observations of the afterglow of GRB000301C,spanning from 1.4 to 350 GHz for the period of 3 to 83 days after the burst.This radio data, in addition to measurements at the optical bands, suggest thatthe afterglow arises from a collimated outflow, i.e. a jet. To test thishypothesis in a self-consistent manner, we employ a global fit and find that amodel of a jet, expanding into a constant density medium (ISM+jet), providesthe best fit to the data. A model of the burst occurring in a wind-shapedcircumburst medium (wind-only model) can be ruled out, and a wind+jet modelprovides a much poorer fit of the optical/IR data than the ISM+jet model. Inaddition, we present the first clear indication that the reported fluctuationsin the optical/IR are achromatic with similar amplitudes in all bands, andpossibly extend into the radio regime. Using the parameters derived from theglobal fit, in particular a jet break time, t_{jet}=7.5 days, we infer a jetopening angle of \theta=0.2, and consequently the estimate of the emittedenergy in the GRB itself is reduced by a factor of 50 relative to the isotropicvalue, giving E=1.1 \times 10^{51} ergs.Comment: 8 pages, ApJ submitte