Cosmic‐Ray Acceleration at Ultrarelativistic Shock Waves: Effects of Downstream Short‐Wave Turbulence

The present paper is the last of a series studying the first-order Fermiacceleration processes at relativistic shock waves with the method of MonteCarlo simulations applied to shocks propagating in realistically modeledturbulent magnetic fields. The model of the background magnetic field structureof Niemiec & Ostrowski (2004, 2006) has been augmented here by alarge-amplitude short-wave downstream component, imitating that generated byplasma instabilities at the shock front. Following Niemiec & Ostrowski (2006),we have considered ultrarelativistic shocks with the mean magnetic fieldoriented both oblique and parallel to the shock normal. For both casessimulations have been performed for different choices of magnetic fieldperturbations, represented by various wave power spectra within a widewavevector range. The results show that the introduction of the short-wavecomponent downstream of the shock is not sufficient to produce power-lawparticle spectra with the "universal" spectral index 4.2. On the contrary,concave spectra with cutoffs are preferentially formed, the curvature andcutoff energy being dependent on the properties of turbulence. Our resultssuggest that the electromagnetic emission observed from astrophysical siteswith relativistic jets, e.g. AGN and GRBs, is likely generated by particlesaccelerated in processes other than the widely invoked first-order Fermimechanism.Comment: 9 pages, 8 figures, submitted to Ap