Microphysical Conditioning of Gamma-Ray Burst Shocks

Sammanfattning: Gamma-Ray Bursts (GRBs) are believed to be connected with either collapse of massive stars or – in some cases – coalescing neutron stars, both of which cause extreme relativistic explosions observable from the entire Universe. GRBs produce rapid flashes of high energy radiation, extending up to 2MeV in photon energies: as much energy per photon as the rest mass of 4 electrons. Bursts may last for many seconds, with millisecond variability, or they may last a mere fraction of a second. The rapid flash is often trailed by an afterglow living on timescales of minutes-to-days after the initial prompt flash has occurred. Late time afterglows are caused mainly by the explosions' interaction with a surrounding circumburst medium (CBM).Here we present results comprising numerical studies of streaming plasmas in relativistic collisionless shocks, and photon-plasma interaction effects, that are believed to be at work in GRBs.Conclusions about the generation of magnetic fields and particle acceleration in the afterglow phase of GRBs are also presented. Momentum anisotropy feeds the shock and create stably growing field structures. These conclusions are tied mainly to processes in afterglow shocks; it may be conjectured that such processes are likely to be present even in earlier phases of the GRB evolution.The instability at work in the very forefront of the afterglow shock is argued not to be the (pure) Weibel-instability, which is induced by thermal anisotropy. Rather, it is a mixed superposition of the filamentation and two-stream instabilities.From simulations, using a novel highly improved particle-in-cell code -- the "photonplasma" code -- that includes sub-Debye sphere processes through Monte-Carlo integration, we futher present results on stochastically induced wakefield processes. In modeling such stochastic effects, induced by Compton scattering of highly anisotropic GRB radiation, we address aspects of the photon-plasma coupling in the GRB-CBM interaction.