Published in Space Science Reviews, Volume 134, Issue 1-4, pp. 71-92, 2008.

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Y. Rephaeli 1, 2, J. Nevalainen 3, T. Ohashi 4, A.M. Bykov 5

1 School of Physics & Astronomy, Tel Aviv University, Tel Aviv, 69978, Israel
2 Center for Astrophysics and Space Sciences, University of California, San Diego, La Jolla, CA 92093-0424
3 Observatory, P.O. Box 14, 00014 University of Helsinki, Helsinki, Finland
4 Department of Physics, Tokyo Metropolitan University, 1-1 Minami-Osawa, Tokyo 192-0397, Japan
5 A.F. Ioffe Institute for Physics and Technology, 194021 St. Petersburg, Russia

Abstract. Recent observations of high energy (> 20 keV) X-ray emission in a few clusters of galaxies broaden our knowledge of physical phenomena in the intracluster space. This emission is likely to be nonthermal, probably resulting from Compton scattering of relativistic electrons by the cosmic microwave background (CMB) radiation. Direct evidence for the presence of relativistic electrons in some 50 clusters comes from measurements of extended radio emission in their central regions. We briefly review the main results from observations of extended regions of radio emission, and Faraday rotation measurements of background and cluster radio sources. The main focus of the review are searches for nonthermal X-ray emission conducted with past and currently operating satellites, which yielded appreciable evidence for nonthermal emission components in the spectra of a few clusters. This evidence is clearly not unequivocal, due to substantial observational and systematic uncertainties, in addition to virtually complete lack of spatial information. If indeed the emission has its origin in Compton scattering of relativistic electrons by the CMB, then the mean magnetic field strength and density of relativistic electrons in the cluster can be directly determined. Knowledge of these basic nonthermal quantities is valuable for the detailed description of processes in intracluster gas and for the origin of magnetic fields.

Key words: Clusters: general, X-ray emission

Table of Contents


Extended IC Emission
Faraday Rotation

Instrumental Requirements
Initial search for NT X-ray emission
Search for NT emission with RXTE
Search for NT emission with BeppoSAX
Search for NT emission with ASCA
Search for NT emission with Suzaku
INTEGRAL observation of the Coma cluster
Limits on cluster gamma-ray emission