Multidimensional simulations of magnetic field amplification and electron acceleration to near-energy equipartition with ions by a mildly relativistic quasi-parallel plasma collision

摘要

The energetic electromagnetic eruptions observed during the prompt phase ofgamma-ray bursts are attributed to synchrotron emissions. The internal shocksmoving through the ultrarelativistic jet, which is ejected by an implodingsupermassive star, are the likely source of this radiation. Synchrotronemissions at the observed strength require the simultaneous presence ofpowerful magnetic fields and highly relativistic electrons. We explore with oneand three-dimensional relativistic particle-in-cell simulations the transitionlayer of a shock, that evolves out of the collision of two plasma clouds at aspeed 0.9c and in the presence of a quasi-parallel magnetic field. The clouddensities vary by a factor of 10. The number densities of ions and electrons ineach cloud, which have the mass ratio 250, are equal. The peak Lorentz factorof the electrons is determined in the 1D simulation, as well as the orientationand the strength of the magnetic field at the boundary of the two collidingclouds. The relativistic masses of the electrons and ions close to the shocktransition layer are comparable as in previous work. The 3D simulation showsrapid and strong plasma filamentation behind the transient precursor. Themagnetic field component orthogonal to the initial field direction is amplifiedin both simulations to values that exceed those expected from the shockcompression by over an order of magnitude. The forming shock isquasi-perpendicular due to this amplification. The simultaneous presence ofhighly relativistic electrons and strong magnetic fields will give rise tosignificant synchrotron emissions.