Published in Space Science Reviews, Volume 134, Issue 1-4, pp. 191-206, 2008.

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Vahé Petrosian 1, Andrei M. Bykov 2 and Yoel Rephaeli 3

1 Department of Applied Physics, Stanford University, Stanford, CA, 94305
Kavli Institute of Particle Astrophysics and Cosmology, Stanford University, Stanford, CA, 94305
2 A.F. Ioffe Institute for Physics and Technology, 194021 St. Petersburg, Russia
3 School of Physics and Astronomy, Tel Aviv University, Tel Aviv, 69978, Israel

Abstract. In this paper we review the possible radiation mechanisms for the observed non-thermal emission in clusters of galaxies, with a primary focus on the radio and hard X-ray emission. We show that the difficulty with the non-thermal, non-relativisitic Bremsstrahlung model for the hard X-ray emission, first pointed out by Petrosian (2001) using a cold target approximation, is somewhat alleviated when one treats the problem more exactly by including the fact that the background plasma particle energies are on average a factor of 10 below the energy of the non-thermal particles. This increases the lifetime of the non-thermal particles, and as a result decreases the extreme energy requirement, but at most by a factor of three. We then review the synchrotron and so-called inverse Compton emission by relativistic electrons, which when compared with observations can constrain the value of the magnetic field and energy of relativistic electrons. This model requires a low value of the magnetic field which is far from the equipartition value. We briefly review the possibilities of gamma-ray emission and prospects for GLAST observations. We also present a toy model of the non-thermal electron spectra that are produced by the acceleration mechanisms discussed in an accompanying paper.

Key words: radiation mechanisms: non-thermal - magnetic fields - galaxies: clusters: general - X-rays: galaxies: clusters

Table of Contents



Bremsstrahlung emission
Emission from relativistic electrons