Published in Space Science Reviews, Volume 134, Issue 1-4, pp. 93-118, 2008.
astro-ph/0801.0985v1

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OBSERVATIONS OF EXTENDED RADIO EMISSION IN CLUSTERS

C. Ferrari ¹, F. Govoni ², S. Schindler ¹, A. M. Bykov ³ and Y. Rephaeli ⁴

¹ Institute for Astro- and Particle Physics, University of Innsbruck, Technikerstr. 25, 6020 Innsbruck, Austria
chiara.ferrari@uibk.ac.at
² INAF - Osservatorio Astronomico di Cagliari, Loc. Poggio dei Pini, Strada 54, 09012 Capoterra, Cagliari, Italy
³ A.F. Ioffe Institute for Physics and Technology, 194021 St. Petersburg, Russia
⁴ School of Physics & Astronomy, Tel Aviv University, Tel Aviv, 69978, Israel

Abstract. We review observations of extended regions of radio emission in clusters; these include diffuse emission in `relics', and the large central regions commonly referred to as `halos'. The spectral observations, as well as Faraday rotation measurements of background and cluster radio sources, provide the main evidence for large-scale intracluster magnetic fields and significant densities of relativistic electrons. Implications from these observations on acceleration mechanisms of these electrons are reviewed, including turbulent and shock acceleration, and also the origin of some of the electrons in collisions of relativistic protons by ambient protons in the (thermal) gas. Improved knowledge of non-thermal phenomena in clusters requires more extensive and detailed radio measurements; we briefly review prospects for future observations.

Key words: Galaxies: clusters: general - Galaxies: intergalactic medium - Radio continuum: general - Radiation mechanisms: non-thermal - Magnetic fields - Acceleration of particles

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