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The prompt emissions of gamma-ray bursts are seeded by radiatingultrarelativistic electrons. Internal shocks propagating through a jet launchedby a stellar implosion, are expected to amplify the magnetic field & accelerateelectrons. We explore the effects of density asymmetry & a quasi-parallelmagnetic field on the collision of plasma clouds. A 2D relativistic PICsimulation models the collision of two plasma clouds, in the presence of aquasi-parallel magnetic field. The cloud density ratio is 10. The densities ofions & electrons & the temperature of 131 keV are equal in each cloud. The massratio is 250. The peak Lorentz factor of the electrons is determined, alongwith the orientation & strength of the magnetic field at the cloud collisionboundary. The magnetic field component orthogonal to the initial plasma flowdirection is amplified to values that exceed those expected from shockcompression by over an order of magnitude. The forming shock isquasi-perpendicular due to this amplification, caused by a current sheet whichdevelops in response to the differing deflection of the incoming upstreamelectrons & ions. The electron deflection implies a charge separation of theupstream electrons & ions; the resulting electric field drags the electronsthrough the magnetic field, whereupon they acquire a relativistic masscomparable to the ions. We demonstrate how a magnetic field structureresembling the cross section of a flux tube grows in the current sheet of theshock transition layer. Plasma filamentation develops, as well as signatures oforthogonal magnetic field striping. Localized magnetic bubbles form. Energyequipartition between the ion, electron & magnetic energy is obtained at theshock transition layer. The electronic radiation can provide a seed photonpopulation that can be energized by secondary processes (e.g. inverse Compton).

astronomy and astrophysics

Magnetic field amplification and electron acceleration to near-energy  equipartition with ions by a mildly relativistic quasi-parallel plasma  protoshock