Wind Interaction Models for the Afterglows of GRB 991208 and GRB 000301C

The simplest model of the afterglows of gamma-ray bursts (GRBs) envisions a spherical blast wave, with a power-law distribution of electron energy above some cutoff, running into a constant-density medium. A refinement involves a narrow jet, often invoked to explain the steep decline and/or steepening of light curves observed in some afterglows. The constant (ambient) density jet model has been applied to GRBs 991208 and 000301C, based to a large extent on radio observations. We show that, for these two sources, a spherical-wind model (with an r -2 density ambient medium) can fit the radio data as well as the jet model. The relatively steep decline and the fairly abrupt steepening of the R -band light curves of, respectively, GRB 991208 and GRB 000301C can be accounted for with a nonstandard, broken power-law distribution of electron energy. Our model predicts a slower late decline for the radio flux than does the jet model.