Properties of relativistic jets in gamma-ray burst afterglows

We extend our calculation of physical parameters of gamma-ray burst (GRB) jets by modeling the broadband emission of the afterglows 970508, 980519, 991208, 000926, 000418, and 010222. Together with 990123, 990510, 991216, and 000301c, there are 10 well-observed afterglows for which the initial opening angle of the GRB jet can be constrained. The jet energies ( after the GRB phase) obtained for this set of afterglows are within one decade around 5 x 10(50) ergs. With the exception of 000418, which requires a jet wider than 1/2 rad, the jet initial half-angle in the other cases ranges from 2degrees to 20degrees. We find that in half of the cases, a homogeneous ambient medium accommodates the afterglow emission better than the windlike r(-2) profile medium expected around massive stars. The two types of media give fits of comparable quality in four cases, with a wind medium providing a better description only for 970508. The circumburst densities we obtain are in the 0.1-100 cm(-3) range, with the exception of 990123, for which it is below 10(-2) cm(3). If in all 10 cases the observed GRB durations are a good measure of the ejecta deceleration time-scale, then the parameters obtained here lead to jet Lorentz factors at the deceleration radius between 70 and 300, anticorrelated with the jet initial aperture, and jet masses around 10(-6) M-.. Our results on the jet energy, opening Lorentz factor, and evacuation of material until breakout provide constraints on theoretical models of GRB jets.