Continued Radio Monitoring of the Gamma‐Ray Burst 991208

We present radio observations of the afterglow of the bright gamma-ray burstGRB 991208 at frequencies of 1.4, 4.9 and 8.5 GHz, taken between two weeks and300 days after the burst. The well-sampled radio light curve at 8.5 GHz showsthat the peak flux density peaked about 10 days after the burst and decayedthereafter as a power-law t^-1.07. This decay rate is more shallow than theoptical afterglow with t^-2.2, which was measured during the first week. Theselate-time data are combined with extensive optical, millimeter and centimetermeasurements and fitted to the standard relativistic blast wave model. Inagreement with previous findings, we find that an isotropic explosion in aconstant density or wind-blown medium cannot explain these broadband datawithout modifying the assumption of a single power-law slope for the electronenergy distribution. A jet-like expansion provides a reasonable fit to thedata. In this case, the flatter radio light curve compared to the optical maybe due to emission from an underlying host galaxy, or due to the blastwavemaking a transition to non-relativistic expansion. The model that bestrepresents the data is a free-form model in which it is assumed that thebroadband emission originates from a synchrotron spectrum, while thetime-evolution of the break frequencies and peak flux density are solved forexplicitly. Although the decay indices for most of the synchrotron parametersare similar to the jet model, the evolution of the cooling break is unusuallyrapid, and therefore requires some non-standard evolution in the shock.(abridged)Comment: ApJ, in pres