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

Gamma-ray burst afterglow flares and rebrightenings of the optical and X-ray light curves 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 hydrodynamic code, AMRVAC, and the synchrotron emission is analysed 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 analysing 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 the recent literature.