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

Abstract

The simplest model of the afterglows of the 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. %%We demonstrate that the 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 non-standard, broken power-law distribution of electron energy. Our model predicts a slower late decline for the radio flux than does the jet model.