Two-shell collisions in the gamma-ray burst afterglow phase

Strong optical and radio flares often appear in the afterglow phase of gamma-ray bursts (GRBs). It has been proposed that colliding ultrarelativistic 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 ultrarelativistic shells in order to explore these events. We examine the case where a cold uniform shell collides with a self-similar Blandford & 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 coupled to a linear radiative transfer code to calculate synchrotron emission. We find steeply rising flares like the behaviour of 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.