Research activities from 2001 to 2011
For many years, the understanding of cosmic ray acceleration processes has become a very complex challenge for high energy astrophysics.
Indeed, many astrophysical sites are supposed to play the role of good accelerators like, e.g., Active Galactic Nuclei, Gamma-Ray Bursts (hereafter GRBs), blazars, supernovae or pulsar winds.
In the standard theory of GRBs, namely the fireball model, particle acceleration processes could be considered at three different stages:
the first one happens at the very beginning of the outflow ejection, consequent upon interactions between the outflow and the magnetized torus surrounding the central black hole.
The second one concerns the acceleration through the mildly relativistic internal shocks, during the adiabatic expansion of the fireball.
At last, the third one takes place when an external ultra-relativistic shocks forms between the outflow and the surrounding interstellar medium.
Since 2001, my research work has mainly been to study capabilities of GRBs to accelerate particles and to produce high energy emission in the internal shock stage.
I focused first on the production of Ultra High Energy Cosmic Rays (UHECRs) via Fermi processes, as it had been suggested by Waxman (1995).
The important point is that Fermi acceleration processes are very dependent on the magnetic turbulence of the medium:
according to previous detailed numerical and theoretical works, rather than using the unphysical Bohm scaling, we chose a more realistic scaling depending on turbulence spectrum of the magnetic field, as in the Kolmogorov theory. Such a description led us to the following conclusion: Gamma-Ray Bursts are unable to produce UHECRs if one considers usual Fermi processes in the internal shocks, because of both too strong synchrotron and expansion losses in the outflow. Nevertheless, the modification of the acceleration process efficiency has some very interesting consequences on the high energy emission from the accelerated electronic population... SEE MY PUBLICATIONS
In another hand, in 2003, we proved (Gialis D. & Pelletier G., A&A, 2004, volume 425, p.395-403) that the generation of UHECRs could be achieved by considering a new scenario of relativistic Fermi process in GRBs during the internal shock stage. Under some conditions, we also found that a synchrotron signature (in GeV range) could be observable with new gamma-ray instruments (like Fermi).
I have also worked on plasma physics in relativistic shocks (instabilities, magnetic reconnection, particule acceleration and magnetized shock structure). For instance, I have studied the efficiency of the relativistic shear acceleration in different outflow models (GRBs, AGNs, pulsar winds...). Including relativistic Fermi acceleration processes in turbulent magnetic field, my new numerical code has provided some interesting results concerning the evolution of a given particle distribution in different shear flows. There is no doubt that very high energy particles could be produced into such a context...
I collaborated with Guy Pelletier (Institut de Planétologie & d'Astrophysique de Grenoble, FRANCE), Frédéric Piron (Laboratoire Univers & Particules de Montpellier, FRANCE) for questions about GRB physics and observations.
For more informations; denis_gialis"at"yahoo.fr
Web address : www.dg-homepage.sup.fr