No visible optical variability from a relativistic blast wave encountering a wind-termination shock

TL;DR

This study uses high-resolution relativistic hydrodynamics simulations to investigate whether a gamma-ray burst's optical afterglow shows variability when encountering a stellar wind termination shock, finding no observable bump in the light curve.

Contribution

It demonstrates that density jumps at wind-termination shocks do not produce optical rebrightenings, resolving a recent controversy and emphasizing the importance of resolving blast wave structures in simulations.

Findings

No bump in light curve even with high density jumps

Reverse shock contribution does not produce observable features

Proper resolution of blast wave structure is crucial for accurate predictions

Abstract

Gamma-ray burst afterglow flares and rebrightenings of the optical and X-ray light curve have been attributed to both late time inner engine activity and density changes in the medium surrounding the burster. To test the latter, we study the encounter between the relativistic blast wave from a gamma-ray burster and a stellar wind termination shock. The blast wave is simulated using a high performance adaptive mesh relativistic hydrodynamics code, AMRVAC, and the synchrotron emission is analyzed in detail with a separate radiation code. We find no bump in the resulting light curve, not even for very high density jumps. Furthermore, by analyzing the contributions from the different shock wave regions we are able to establish that it is essential to resolve the blast wave structure in order to make qualitatively correct predictions on the observed output and that the contribution from the reverse shock region will not stand out, even when the magnetic field is increased in this region by repeated shocks. This study resolves a controversy in recent literature.