2001ApJ...556.1002P

The parameters of the gamma-ray burst (GRB) 000301c afterglow are determined within the usual framework of synchrotron emission from relativistic ejecta, through fits to the available radio-to-optical data. It is found that (1) the jet energy after the GRB phase is ∼3x10⁵⁰ ergs, (2) the initial opening angle of the jet is ∼13°, (3) the medium that decelerates the afterglow has a density ∼25 cm^–3, (4) the power-law distribution of the shock-energized electrons is hard, with an index around 3/2, and (5) the cooling frequency was located below the optical domain. Furthermore, we find that the collimation of ejecta alone cannot explain the sharp and large magnitude break observed in the R-band emission of this afterglow at a few days, and that a high-energy break in the electron distribution, corresponding to an electron energy close to equipartition, together with the lateral spreading of the jet accommodate better this break. Microlensing by a star in an intervening galaxy produces only a flattening of the afterglow emission and cannot explain the mild brightening exhibited by the optical emission of 000301c at ∼4 days.