Illya Plotnikov, Guy Pelletier, Martin Lemoine
Collisionless relativistic shocks have been the focus of intense theoretical and numerical investigations and these interesting physics have a direct impact on the generation of energetic particles and the interpretation of gamma ray spectra. The Fermi acceleration process that takes place in these shocks is intimately linked with the excitation of micro-turbulence responsible for the shock formation, electron heating and supra-thermal tail generation that in turn excites micro-turbulence, developing thus a self-sustaining phenomenon. In this paper we discuss the development of the micro-turbulence and we investigate two important issues: firstly the transport of supra-thermal particles in the excited microturbulence upstream of the shock and its consequences for the efficiency of the Fermi process; secondly, the preheating process of the incoming background electrons as they cross the shock precursor and experience relativistic oscillations in the electric field of the micro-turbulence.We emphasize the importance of the motion of the electromagnetic disturbances relatively to the background plasma and to the shock front. The investigation is carried out for the two major instabilities involved in the precursor of relativistic shocks, the filamentation instability and the oblique two stream instability. Bearing in mind these new results, we analyze the performance of the Fermi acceleration process in various high energy sources and especially in the termination shock of gamma-ray bursts. We emphasize the high efficiency of a collisionless ultra-relativistic shock for accelerating electrons and radiating a synchrotron-like photon spectrum up to several GeV.
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http://arxiv.org/abs/1206.6634
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