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Speaker |
PELLETIER, Guy |
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Co-autors |
Martin Lemoine |
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Talk Title |
Fermi Acceleration at Relativistic Shocks, Generation of Electromagnetic Turbulence and Performances |
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Abstract |
After promising results obtained at the beginning of the century (Gallant & Achterberg 1999, Achterberg et al 2001, Lemoine & Pelletier 2003) where the formation of a power law energy spectrum with an universal index has been predicted with numerical and semi analytical works, some difficulty arised. In 2006, Niemiec et al. and M. Lemoine & G.Pelletier found independently, with different methods, that an oblique magnetic mean field inhibits the Fermi cycles. Moreover we have shown that a turbulent cascade with a large scale correlation length had the same effect. Thus in order to make Fermi process operative at relativistic shocks we have shown that it is necessary that an intense turbulence at short scale be generated. Observations of Gamma Ray Bursts indicate that, like in Supernova Remnants, suprathermal electrons are accelerated to very high energies, that they display a power law energy distribution and that magnetic fluctuations are considerably amplified. The relevant instabilities that are able to produce the expected electromagnetic turbulence have been investigated (by several authors including ourself) together with the collisionless shock structure and are likely allowing Fermi acceleration of electrons if the ambient medium has a low magnetization. The possible generation of UHE Cosmic rays at GRB shocks is reconsidered; conditions for its success will be indicated. |
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