Properties of Gamma-Ray Burst Jets Obtained from Afterglow Modelling

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Abstract
We model the broadband emission and determine the basic characteristics of eight GRB afterglows whose features allow the determination of the burst collimation. The resulting post‐GRB jet energies are consistent with a universal value of 3 × 1050 erg. The jet initial apertures are in the 2–14 degree range, half of the jets being narrower than 3 degrees. We find that homogeneous circumburst media are consistent with the emission of all these afterglows while, with a couple of exceptions, wind‐like density profiles are not. The external densities we find are typically in the 0.1–50 particles per cc range. For all eight afterglows the total energy in shock‐accelerated electrons is close to equipartition with protons. However the slope of the power‐law electron distribution is not universal, varying between 1.4 and 2.8. From the jet parameters in the afterglow phase we infer initial bulk Lorentz factors between 70 and 300. Our results on the jet energy, opening and Lorentz factor provide constraints on theoretical models of GRB jets.

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