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Astron. Astrophys. 332, 395-409 (1998)

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1. Introduction

Large extended sources of synchrotron emission are found in some clusters of galaxies: radio halos and radio relics (Fig. 1 ; for reviews, see Jaffe 1992; Feretti & Giovannini 1996). The rare cluster radio halo phenomena are large regions of diffuse unpolarized radio emission, centrally located, and have radio structures roughly similar to that of the thermal X-ray emission (Deiss et al. 1997). The so-called radio relics are peripherally located, and are more irregularly shaped polarized radio sources. Radio relics are believed to be the remnants of radio lobes of radio galaxies, where the former active galaxy has become inactive or has moved away.

[FIGURE] Fig. 1. Westerbork Synthesis Radio Telescope map at 327 MHz of the Coma cluster from Giovannini et al. (1991). The central halo source Coma C and the cluster relic 1253+275 are identified. Capital letters indicate some extended Coma cluster galaxies, as following: A = NGC 4839, B = NGC 4827, C = NGC 4789. 10' corresponds to 400 kpc [FORMULA].

For radio halos a number of candidate sources for the relativistic, synchrotron emitting electrons were discussed in the literature (Schlickeiser et al. 1987 and references therein). Yet, the source of the energetic electrons in the relics is unclear, due to the shortness of the electron cooling time compared to the time since electron injection from a galaxy at the location of the relic was possible. Since the radio spectra of relics are steep but frequently do not exhibit any cutoff in the observed range, an efficient particle acceleration mechanism has to be present within or close to them. We show that shocks of the large-scale gas motion are expected at the typical peripheral locations of relics, either resulting from cluster mergers, or from steady state accretion shocks by gas falling into the cluster potential. Kang et al. (1997) showed that protons might be accelerated in cluster accretion shocks to energies comparable with the most energetic cosmic ray events observed at earth. Also electrons should be accelerated and become visible by radio emission at locations where magnetic fields are present. Therefore the magnetized plasma, accumulated behind the shock or left behind as a remnant of a radio galaxy, could furnish this acceleration region and should also increase the efficiency of the acceleration mechanism. We demonstrate that for the case of the radio relic 1253+275 close to Coma A (Fig. 1), the observed spectral index and polarization of the radio emission agree well with the prediction for an accretion shock. Eight other relics are also briefly discussed.

We adopt for the following [FORMULA] and [FORMULA].

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© European Southern Observatory (ESO) 1998

Online publication: March 23, 1998