Untitled - Laboratoire d'Astrophysique de l'Observatoire de Grenoble

Excitation of turbulence by Cosmic Rays upstream a shock and consequences for the transport and the Fermi
acceleration.
The results we obtained on the transport of Cosmic Rays make its dependence on the MHD turbulence
spectrum precise. There exist semi-phenomenological theories of inertial turbulence spectra for common situations
where the turbulence is excited at large scales and then cascades over several decades towards smaller
scales where the dissipation takes place. Moreover the Reynolds numbers in astrophysical media are very large.
However, the turbulence excited upstream astrophysical shocks is of a particular nature. Indeed it is excited
by an instability resulting from the acceleration of Cosmic Rays at the shock and the instability amplifies the
MHD modes over all scales through a resonnant regime (larger scales) and a non-resonnant regime (shorter
scales). We have thus undergone the calculation of the turbulence spectra and the transport coefficients. Then
we examined the consequences of the excitation of MHD turbulence on the energetic balance and the final
spectrum of the Cosmic Rays that is steepened. This theoretical effort was made necessary by the indications
delivered by the new observations of supernovae remnants by Chandra and XMM. Our theory incorporates not
only the newest developments on anisotropic MHD turbulence but also an important effect: the backscattering
of progressive Alfvén modes off sound waves (more precisely the slow magneto-sonic waves). These studies have
been realized with Martin Lemoine, Alexandre Marcowith (CESR) and G.P. (in course of publication).
High energy emission and Cosmic Rays from Gamma-Ray Bursts
Gamma-Ray Bursts (GRBs) are phenomena radiating as much energy as supernovae but in a very short
time, a few seconds or even less. The collimation of the ultra relativistic flows involved in GRBs strongly
amplifies the energy flux. Under the less favorable hypothesis about the magnetic field and its irregularities, it
has been shown that Cosmic Rays undergoing scattering off relativistic magnetized fronts (revealed by the light
curves) could indeed reach the UHE range (Gialis & Pelletier 2004). The performances of GRBs as sources of
high energy radiation in the form of photons and neutrinos have been estimated (Gialis & Pelletier 2005). A
gamma diagnosis of UHECR generation is proposed; the diagnosis in the range of a few tens of GeV appears
to be non-ambiguous, because this signal of hadronic origin should not be contaminated by inverse Compton
emission by electrons which is deeply in the Klein-Nishina regime. We are eagerly waiting for the observations
of GRBs by GLAST that could make this diagnosis.
15.7 Heavy numerical simulations
The field of theoretical astrophysical dynamics has undergone two major evolutions in the last two decades or
so. First, the idea that magnetic fields play a key role has progressively imposed itself to a community in which
MHD phenomena had long been regarded as exotic. Secondly, the progressive rise of powerful numerical tools
(both software and hardware) has transformed numerical simulations from a gadget to a must in the exploration
and understanding of the nonlinear outcome of the physical processes considered as important in our trade.
Consequently, MHD has become the central area of study of accretion-ejection related phenomena, and the
modern dynamicist working in this field must not only master analytical techniques, but numerical ones as well.
To a lesser but growing extent, a similar evolution can be witnessed in the field of high energy astrophysical
processes, which is also of direct interest for the team activity.
The French community is still largely lagging behind the international leadership on the numerical front, by
lack of both human and material means (despite the information made by the ASSNA). The SHERPA team
has become more and more aware of this deficiency for its own activity, and has therefore put a particular
emphasis on numerics by progressively building up an internal expertise, and by defining as a goal to address
an evergrowing fraction of problems through (M)HD simulations (in support of more theoretical analyzes). In
practice, this numerical activity is only now reaching a production stage, and has been undertaken in several
directions:
i- (M)HD stability and turbulent transport in disks and jets with Geoffroy Lesur (PhD started september
2004) and P.-Y. Longaretti.
ii- 2D magnetospheric star/disk interaction with Nicolas Bessolaz (PhD started november 2004) and J.
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