Nonlinear particle acceleration in relativistic shocks. Astroparticle Phys

Abstract

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, γ0 � 3, can have compression ratios considerably greater than 3 and the momentum distribution of energetic particles no longer follows a power law relation. These results may be important for the interpretation of gammaray bursts if mildly relativistic internal and/or afterglow shocks play an important role accelerating particles that produce the observed radiation. For γ0 � 10, r approaches 3 and the so-called ‘universal ’ test-particle result of N(E) ∝ 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. Subject headings: Cosmic rays — acceleration of particles — relativistic shock waves — gamma-ray bursts; PACS: 52.60, 96.40 1