No early time flares from GRB afterglows blast waves encountering sudden circumburst density change

Author: Ilana Gat

Abstract: Afterglows of gamma-ray bursts have been observed and studied for more than fifteen years and generally produce a light curve with the flux following a power law evolution in time. However, recent observations have shown large flares in light curves at early times. One possible explanation for these flares is the interaction of a relativistic blast wave with a circumburst density transition. We model this type of interaction computationally in one and two dimensions and analytically in one dimension. We simulate a blast wave traveling in a stellar wind environment that encounters a sudden change in density, followed by a homogeneous medium. We find that not only are early time flares not observable for an encounter with a sudden density increase, such as a wind termination shock, but early time flares are not produced for an encounter with a sudden density decrease either. Furthermore, by extending our analysis to two dimensions, we are able to resolve the spreading, collimation, and edge effects of the blast wave as it encounters the change in circumburst medium. In all cases, we find that an early time flare will not be produced for any type of density change.