Two-shell collisions in the GRB afterglow phase
  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.
