EDP Sciences
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Volume 394, Number 3, November II 2002
Page(s) 1141 - 1144
Section Physical and chemical processes
DOI http://dx.doi.org/10.1051/0004-6361:20021173

A&A 394, 1141-1144 (2002)
DOI: 10.1051/0004-6361:20021173

Comment on the first-order Fermi acceleration at ultra-relativistic shocks

M. Ostrowski1, 2 and J. Bednarz1

1  Obserwatorium Astronomiczne, Uniwersytet Jagiellonski, ul. Orla 171, 30-244 Kraków, Poland
2  Institut für Theoretische Physik, Lehrstuhl IV: Weltraum und Astrophysik, Ruhr-Universität Bochum, Germany

(Received 17 April 2002 / Accepted 23 July 2002 )

The first-order Fermi acceleration process at an ultra-relativistic shock wave is expected to create a particle spectrum with the unique asymptotic spectral index $\sigma_{\gamma \,\gg\, 1} \approx 2.2$. Below, we discuss this result and differences in its various derivations, which - explicitly or implicitly - always require highly turbulent conditions downstream of the shock. In the presence of medium amplitude turbulence the generated particle spectrum can be much steeper than the above asymptotic one. We also note problems with application of the pitch angle diffusion model for particle transport near the ultra-relativistic shocks.

Key words: acceleration of particles -- shock waves -- X-rays: bursts -- gamma-rays: bursts -- ISM: cosmic rays

Offprint request: M. Ostrowski, mio@oa.uj.edu.pl

© ESO 2002