The rise of the afterglow in GRB 050820a - AstrophysicsReport as inadecuate

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Abstract: The early optical afterglow of GRB 050820a recorded by the RAPTOR telescopeshows both a contribution from the prompt emission and the initial rise of theafterglow. It is therefore well-suited for the study of the dynamical evolutionof the GRB ejecta when it first undergoes the decelerating effect of theenvironment. This is a complex phase where the internal, reverse, and forwardshocks can all be present simultaneously. We have developed a simplified modelthat can follow these different shocks in an approximate, but self-consistentway. It is applied to the case of GRB 050820a to obtain the prompt andafterglow light curves. We show that the rise of the afterglow during thecourse of the prompt emission has some important consequences. The reverseshock propagates back into the ejecta before internal shocks are completed,which affects the shape of the gamma-ray profile. We get the best results whenthe external medium has a uniform density, but obtaining a simultaneous fit ofthe prompt and afterglow emission is not easy. We discuss a few possibilitiesthat could help to improve this situation.

Author: F. Genet, F. Daigne, R. Mochkovitch


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