The bright gamma-ray burst 991208: Tight constraints on afterglow models from observations of the early-time radio evolution

The millimeter wavelength emission from GRB 991208 is the second brightest ever detected, yielding a unique data set. We present here well-sampled spectra and light curves over more than two decades in frequency for a 2 week period. This data set has allowed us for the first time to trace the evolution of the characteristic synchrotron self-absorption frequency ν_a, peak frequency ν_m, and the peak flux density F_m; we obtain ν_a ∝ t^-0.15±0.23, ν_m ∝ t^-1.7±0.7, and F_m ∝ t^-0.47±0.20. From the radio data we find that models of homogeneous or wind-generated ambient media with a spherically symmetric outflow can be ruled out. A model in which the relativistic outflow is collimated (a jet) can account for the observed evolution of the synchrotron parameters, the rapid decay at optical wavelengths, and the observed radio-to-optical spectral flux distributions that we present here, provided that the jet transition has not been fully completed in the first 2 weeks after the event. These observations provide additional evidence that rapidly decaying optical/X-ray afterglows are due to jets and that such transitions either develop very slowly or perhaps never reach the predicted asymptotic decay F(t) ∝ t^-P.