Journal
ASTROPARTICLE PHYSICS
Volume 18, Issue 3, Pages 213-228Publisher
ELSEVIER SCIENCE BV
DOI: 10.1016/S0927-6505(02)00142-1
Keywords
cosmic rays; acceleration of particles; relativistic shock waves; gamma-ray bursts
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Monte Carlo techniques are used to model nonlinear particle acceleration in parallel collisionless shocks of various speeds, including mildly relativistic ones. When the acceleration is efficient, the backreaction of accelerated particles modifies the shock structure and causes the compression ratio, r, to increase above test-particle values. Modified shocks with Lorentz factors, gamma(0) less than or similar to 3, can have compression ratios considerably >3 and the momentum distribution of energetic particles no longer follows a power law relation. These results may be important for the interpretation of gamma-ray bursts if mildly relativistic internal and/or afterglow shocks play an important role accelerating particles that produce the observed radiation. For gamma(0) greater than or similar to 10, r approaches 3 and the so-called 'universal' test-particle result of N(E) proportional to E-(2.3) is obtained for sufficiently energetic particles. In all cases, the absolute normalization of the particle distribution follows directly from our model assumptions and is explicitly determined. (C) 2002 Elsevier Science B.V. All rights reserved.
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