The Early Afterglow as a Diagnostic Tool for GRB Outflows

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 it (analytically and numerically) focusing on the reverse shock emission. We show that: (i) The strength of the reverse shock optical flash depends strongly on the properties of the relativistic ejecta. (ii) Previous calculations have generally significantly overestimated the strength of this optical flash. (iii) Detailed observations of the early afterglow optical light curve alone 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