The Early Afterglow as a Diagnostic Tool for GRB Outflows

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  • 21 March 2004
Abstract
The Gamma-Ray burst (GRB) - afterglow transition is one of the most interesting and least studied GRB phases. During this phase the relativistic ejecta begins interacting with the surrounding matter. A strong short lived reverse shock propagates into the ejecta (provided that it is Baryonic) while the forward shock begins to shape the surrounding matter into a Blandford-McKee profile. We suggest a parametrization of the early afterglow light curve and we calculate (analytically and numerically) the parameters, focusing on the reverse shock emission. We present a new test of the relation between the radio and the optical emission to confirm that the emission results from a reverse shock. We show that: (i) The reverse shock optical flash depends strongly on initial conditions of the relativistic ejecta.(ii) Previous calculations have generally overestimated the strength of this optical flash. (iii) Detailed observations of the early afterglow light curve would enable us to determine the initial physical conditions within the relativistic ejecta and specifically to determine its constitution (Baryonic vs. Poynting flux) and to estimate its Lorentz factor and its width.

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