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Issue A&A Volume 439, Number 2, August IV 2005 Page(s) 461 - 464 Section Astrophysical processes DOI 10.1051/0004-6361:20053349

A&A 439, 461-464 (2005)
DOI: 10.1051/0004-6361:20053349

Research Note

Particle acceleration in thick parallel shocks with high compression ratio

J. J. P. Virtanen¹ and R. Vainio<sup>2

1</sup>  Tuorla Observatory, Väisälä Institute for Space Physics and Astronomy, Väisäläntie 20, 21500 Piikkiö, Finland
    e-mail: joni.virtanen@utu.fi
²  Department of Physical Sciences, PO Box 64, 00014 University of Helsinki, Finland
    e-mail: rami.vainio@helsinki.fi

(Received 2 May 2005 / Accepted 1 June 2005 )

Abstract
We report studies on first-order Fermi acceleration in parallel modified shock waves with a large scattering center compression ratio expected from turbulence transmission models. Using a Monte Carlo technique we have modeled particle acceleration in shocks with a velocity ranging from nonrelativistic to ultrarelativistic and a thickness extending from nearly steplike to very wide structures exceeding the particle diffusion length by orders of magnitude. The nonrelativistic diffusion approximation is found to be surprisingly accurate in predicting the spectral index of a thick shock with large compression ratio even in the cases involving relativistic shock speeds.

Key words: acceleration of particles -- shock waves -- cosmic rays

© ESO 2005

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