Two-shell collisions in the GRB afterglow phase

TL;DR

This paper uses high-resolution simulations to study collisions of ultra-relativistic shells in GRB afterglows, revealing how such events produce optical and radio flares influenced by jet geometry and synchrotron self-absorption.

Contribution

It provides detailed numerical modeling of shell collisions in GRB afterglows, exploring the effects of jet angle and synchrotron self-absorption on flare characteristics.

Findings

Steep flare rises for small jet angles

Gradual rebrightenings for large jet angles

Radio flare shape affected by synchrotron self-absorption

Abstract

Strong optical and radio flares often appear in the afterglow phase of Gamma-Ray Bursts (GRBs). It has been proposed that colliding ultra-relativistic shells can produce these flares. Such consecutive shells can be formed due to the variability in the central source of a GRB. We perform high resolution 1D numerical simulations of late collisions between two ultra-relativistic shells in order to explore these events. We examine the case where a cold uniform shell collides with a self-similar Blandford and McKee shell in a constant density environment and consider cases with different Lorentz factor and energy for the uniform shell. We produce the corresponding on-axis light curves and emission images for the afterglow phase and examine the occurrence of optical and radio flares assuming a spherical explosion and a hard-edged jet scenario. For our simulations we use the Adaptive Mesh Refinement version of the Versatile Advection Code (AMRVAC) coupled to a linear radiative transfer code to calculate synchrotron emission. We find steeply rising flare like behavior for small jet opening angles and more gradual rebrightenings for large opening angles. Synchrotron self-absorption is found to strongly influence the onset and shape of the radio flare.