No flares from Gamma-Ray Burst afterglow blast waves encountering sudden circumburst density change

TL;DR

This study models gamma-ray burst afterglow interactions with sudden density changes in the surrounding medium, finding that such encounters do not produce observable flares, contrary to some previous hypotheses.

Contribution

The paper provides computational and analytical models showing that abrupt density transitions in the circumburst environment do not generate observable afterglow flares.

Findings

Flares are not produced by sudden density increases.

Flares are not produced by sudden density decreases.

Two-dimensional simulations reveal blast wave spreading and edge effects.

Abstract

Afterglows of gamma-ray bursts are observed to produce light curves with the flux following power law evolution in time. However, recent observations reveal bright flares at times on the order of minutes to days. One proposed explanation for these flares is the interaction of a relativistic blast wave with a circumburst density transition. In this paper, we model this type of interaction computationally in one and two dimensions, using a relativistic hydrodynamics code with adaptive mesh refinement called RAM, 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, and compute the observed radiation using a synchrotron model. We show that flares are not observable for an encounter with a sudden density increase, such as a wind termination shock, nor for an encounter with a sudden density decrease. 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 considered in this paper, we find that a flare will not be observed for any of the density changes studied.