Particle acceleration by ultrarelativistic shocks: theory and simulations期刊界 All Journals 搜尽天下杂志传播学术成果专业期刊搜索期刊信息化学术搜索

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Particle acceleration by ultrarelativistic shocks: theory and simulations

Authors:	Abraham Achterberg Yves A Gallant John G Kirk Axel W Guthmann

Institution:	¹Astronomical Institute, Utrecht University, PO Box 80 000, 3508 TA Utrecht, the Netherlands;²Center for High Energy Astrophysics, Kruislaan 403, 1098 SJ Amsterdam, the Netherlands;³Max-Planck Institut für Kernphysik, Postfach 10 39 80, 69029 Heidelberg, Germany

Abstract:	We consider the acceleration of charged particles near ultrarelativistic shocks, with Lorentz factor . We present simulations of the acceleration process and compare these with results from semi-analytical calculations. We show that the spectrum that results from acceleration near ultrarelativistic shocks is a power law, , with a nearly universal value for the slope of this power law. We confirm that the ultrarelativistic equivalent of the Fermi acceleration at a shock differs from its non-relativistic counterpart by the occurrence of large anisotropies in the distribution of the accelerated particles near the shock. In the rest frame of the upstream fluid, particles can only outrun the shock when their direction of motion lies within a small loss cone of opening angle around the shock normal. We also show that all physically plausible deflection or scattering mechanisms can change the upstream flight direction of relativistic particles originating from downstream by only a small amount: . This limits the energy change per shock crossing cycle to , except for the first cycle where particles originate upstream. In that case the upstream energy is boosted by a factor for those particles that are scattered back across the shock into the upstream region.

Keywords:	acceleration of particles shock waves cosmic rays gamma-rays: bursts

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