The rise of the afterglow in GRB 050820a

TL;DR

This paper models the early optical afterglow of GRB 050820a, revealing how reverse shocks influence the prompt emission and afterglow, and highlights the challenges in fitting observational data with current models.

Contribution

It introduces a simplified, self-consistent model for the complex shock interactions during the early afterglow phase of GRBs, applied specifically to GRB 050820a.

Findings

Reverse shock affects gamma-ray profile shape.

Best fit achieved with uniform external medium.

Simultaneous prompt and afterglow fit remains challenging.

Abstract

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