Continued Radio Monitoring of the Gamma-Ray Burst 991208

We present radio observations of the afterglow of the bright γ-ray burst GRB 991208 at frequencies of 1.4, 4.9, and 8.5 GHz, taken between two weeks and 300 days after the burst. The well-sampled radio light curve at 8.5 GHz shows that the flux density peaked about 10 days after the burst and decayed thereafter as a power law FR ∝ t-1.07±0.09. This decay rate is more shallow than the optical afterglow of GRB 991208 with F0 ∝ t-2.2, which was measured during the first week. These late-time data are combined with extensive optical, millimeter, and centimeter measurements and fitted to the standard relativistic blast wave model. In agreement with previous findings, we find that an isotropic explosion in a constant-density or wind-blown medium cannot explain these broadband data without modifying the assumption of a single power-law slope for the electron energy distribution. A jetlike expansion provides a reasonable fit to the data. In this case, the flatter radio light curve compared to the optical may be due to emission from an underlying host galaxy, or due to the blast wave making a transition to nonrelativistic expansion. The model that best represents the data is a free-form model in which it is assumed that the broadband emission originates from a synchrotron spectrum, while the time evolution of the break frequencies and peak flux density are solved for explicitly. Although the decay indices for most of the synchrotron parameters are similar to those for the jet model, the evolution of the cooling break is unusually rapid (νc ∝ t-2) and therefore requires some nonstandard evolution in the shock.