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Comptes Rendus Physique
Volume 16, n° 6-7
pages 628-640 (août 2015)
Doi : 10.1016/j.crhy.2015.08.012
On the origin of very-high-energy photons in astrophysics: A short introduction to acceleration and radiation physics
De l'origine des photons de très haute énergie en astrophysique : une brève introduction aux mécanismes d'accélération et de rayonnement

Martin Lemoine a, , Guy Pelletier b
a Institut d'astrophysique de Paris, CNRS–UPMC, 98 bis, boulevard Arago, 75014 Paris, France 
b Institut de planétologie et d'astrophysique de Grenoble, UJF–Université Grenoble-1/CNRS–INSU, 34, rue de la Piscine, 38000 Grenoble, France 

Corresponding author.

Powerful astrophysical sources produce non-thermal spectra of very-high-energy photons, with generic power-law distributions, through various radiative processes of charged particles, e.g., synchrotron radiation, inverse Compton processes, and hadronic interactions. Those charged particles have themselves been accelerated to ultra-relativistic energies in intense electromagnetic fields in the source. In many cases, the exact acceleration scheme is not known, but standard scenarios, such as Fermi mechanisms and reconnection processes are generally considered as prime suspects for the conversion of bulk kinetic or electromagnetic energy into a power law of supra-thermal particles. This paper proposes a short introduction to the various acceleration and radiative processes which shape the distributions of very-high-energy photons ( ) in astrophysics.

The full text of this article is available in PDF format.

Les sources astrophysiques puissantes sont à l'origine de spectres non thermiques de photons de très haute énergie, généralement caractérisés par des distributions en loi de puissance. Ces photons sont le fruit de processus radiatifs divers de particules chargées primaires, qui interagissent, par exemple, par rayonnement synchrotron, processus Compton inverse ou par des interactions hadroniques. Ces particules chargées ont, quant à elles, été accélérées à des énergies ultra-relativistes dans les champs électromagnétiques intenses des sources. Le mécanisme exact de l'accélération est bien souvent inconnu, mais les processus de Fermi, ou de reconnexion, sont généralement considérés comme des agents idéaux d'une conversion d'énergie d'ensemble, cinétique ou electromagnétique, en énergie de particules supra-thermiques. Cet article propose une brève introduction à la physique de ces processus d'accélération et de rayonnement, à la source des spectres de photons de très haute énergie ( ) en astrophysique.

The full text of this article is available in PDF format.

Keywords : Particle acceleration, Cosmic rays, Gamma-rays, High-energy astrophysics

Mots-clés : Accélération de particules, Rayons cosmiques, Photons gamma, Astrophysique des hautes énergies

1  Throughout this article, we use the index notation, e.g.,  , to indicate that the quantity is expressed in a given frame, here  .
2  Actually, the condition to return downstream is  , where   denotes the velocity of the shock relative to upstream.
3  As well, the exact condition is   so that the particle can outrun the shock, which is moving with velocity   relative to downstream.
4  In principle, escape does not occur upstream, since the upstream plasma is advected at the shock velocity towards the shock front, as viewed in the shock rest frame. However, the highest energy particles, which may explore the far upstream regions beyond the shock precursor, where the turbulence level is low, may decouple from the flow and escape upstream.

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