Properties of Relativistic Jets in Eight Gamma-Ray Burst Afterglows

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  • 7 September 2001
Abstract
We extend our calculation of physical parameters of GRB afterglows through modelling of their broadband emission to three other cases: 980519, 000926, and 010222. Together with 990123, 990510, 991208, 991216, and 000301c, there are eight afterglows whose optical and radio emission allow determination of the burst collimation. The jet energies (after the GRB phase) obtained for this sample of eight afterglows are consistent with a universal value, ~3E50 erg, despite a relatively broad distribution of the jet initial half-angle (2-14 deg). We find that homogeneous external media are consistent with the emission of all these afterglows while, with a couple of exceptions, wind density profiles are incompatible with the observed multi-wavelength light-curves. The circumburst densities we found are in the 0.1-50 per cc range with the exception of 990123 (and 980703), for which this density is below 0.01 per cc. This suggests that, if GRBs are due to collapsars, the wind expelled by the GRB progenitor is rather weak and the circumburst environment is associated with the superbubbles formed by clusters of massive stars. If for all eight cases the observed GRB durations are close to the ejecta deceleration timescale, then the parameters obtained here lead to jet initial bulk Lorentz factors between 70 and 300 and jet masses around 1E-6 M_sun, implying that the initial baryonic load of the jet is 1E-5 - 1E-4 of the mass of the GRB progenitor contained within the jet aperture. Our results on the jet energy, opening, Lorentz factor, and evacuation of material until break-out provide constraints on theoretical models of GRB jets.

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