Flaring up: radio diagnostics of the kinematic, hydrodynamic and environmental properties of gamma-ray bursts

Abstract

The specific incidence of radio flares appears to be significantly larger than that of the prompt optical emission. This abundance, coupled with the reverse shock interpretation, suggests that radio flares add a unique probe on the physics of gamma-ray burst (GRB) shocks. Motivated thus, we estimate the strength of the reverse shock expected for bursts in which multiwavelength observations have allowed the physical parameters of the forward shock to be determined. We use all six bursts (980519, 990123, 990510, 991208, 991216, 000418) which are found to be adiabatic and thus are predicted to have a strong reverse shock. We aim to constrain the hydrodynamic evolution of the reverse shock and the initial bulk Lorentz factor — which we found to be between 10² and 10³ and well above the lower limits derived from the requirement that GRBs be optically thin to high-energy photons. In half of the cases we improve the description of the early afterglow light curves by adding a contribution from the reverse shock. Modelling of this early emission provides the opportunity to investigate the immediate surroundings of the burst. For 991216 and 991208, the expected 1/r² density structure for a stellar wind is not compatible with the early afterglow light curves. Considering the radial range relevant to these GRBs, we discuss the conditions under which the inclusion of a wind termination shock may resolve the absence of a 1/r² density profile.

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