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 spe ctra 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 evolut ion of the characteristic synchrotron self-absorption frequency v(a), peak frequency v(m), and the peak flux density F-m; we obtain v(a) proportional to t(-0.15+/-0.23), v(m) proportional to t(-1.7+/-0.7), and F-m proportiona l to t(-0.47+/-0.20). From the radio data we find that models of homogeneou s or wind-generated ambient media with a spherically symmetric outflow can be ruled out. A model in which the relativistic outflow is collimated (a je t) can account for the observed evolution of the synchrotron parameters, th e rapid decay at optical wavelengths, and the observed radio-to-optical spe ctral flux distributions that we present here, provided that the jet transi tion has not been fully completed in the first 2 weeks after the event. The se observations provide additional evidence that rapidly decaying optical/X -ray afterglows are due to jets and that such transitions either develop ve ry slowly or perhaps never reach the predicted asymptotic decay F(t) propor tional to t(-p).