Aims. The aim of the present work is to constrain the Coma cluster magnetic field strength, its radial profile and power spectrum by comparing Faraday rotation measure (RM) images with numerical simulations of the magnetic field. Methods. We have analyzed polarization data for seven radio sources in the Coma cluster field observed with the Very Large Array at 3.6, 6 and 20 cm, and derived Faraday rotation measures with kiloparsec scale resolution. Random three dimensional magnetic field models have been simulated for various values of the central intensity B 0 and radial power-law slope η, where η indicates how the field scales with respect to the gas density profile. Results. We derive the central magnetic field strength, and radial profile values that best reproduce the RM observations. We find that the magnetic field power spectrum is well represented by a Kolmogorov power spectrum with minimum scale ∼2 kpc and maximum scale ∼34 kpc. The central magnetic field strength and radial slope are constrained to be in the range (B 0 = 3.9 μG; η = 0.4) and (B 0 = 5.4 μG; η = 0.7) within 1σ. The best agreement between observations and simulations is achieved for B 0 = 4.7 μG; η = 0.5. Values of B 0 > 7 μG and <3 μG as well as η < 0.2 and η > 1.0 are incompatible with RM data at 99% confidence level.

Abstract. We present a numerical approach to investigate the relationship between magnetic fields and Faraday rotation effects in clusters of galaxies. We can infer the structure and strength of intra-cluster magnetic fields by comparing our simulations with the observed polarization properties of extended cluster radio sources such as radio galaxies and halos. We find the observations require a magnetic field which fluctuates over a wide range of spatial scales (at least one order of magnitude). If several polarized radio sources are located at different projected positions in a galaxy cluster, as is the case for A119, detailed Faraday rotation images allow us to constrain both the magnetic field strength and the slope of the power spectrum. Our results show that the standard analytic expressions applied in the literature overestimate the cluster magnetic field strengths by a factor of ∼2. We investigate the possible effects of our models on beam depolarization of radio sources whose radiation traverses the magnetized intracluster medium. Finally, we point out that radio halos may provide important information about the spatial power spectrum of the magnetic field fluctuations on large scales. In particular, different values of the index of the power spectrum produce very different total intensity and polarization brightness distributions.

Aims. We present the first results of an ongoing project devoted to the search of giant radio halos in galaxy clusters located in the redshift range z=0.2-0.4. One of the main goals of our study is to measure the fraction of massive galaxy clusters in this redshift interval hosting a radio halo, and to constrain the expectations of the particle re-acceleration model for the origin of non-thermal radio emission in galaxy clusters. Methods. We selected 27 REFLEX clusters and here we present Giant Metrewave Radio Telescope (GMRT) observations at 610 MHz for 11 of them. The sensitivity (1σ) in our images is in the range 35-100 µJy beam −1 for all clusters. Results. We found three new radio halos, doubling the number of halos known in the selected sample. In particular, giant radio halos were found in A 209 and RXCJ 2003.5-2323, and one halo (of smaller size) was found in RXCJ 1314.4-2515. Candidate extended emission on smaller scale was found around the central galaxy in A 3444 which deserves further investigation. Furthermore, a radio relic was found in A 521, and two relics were found in RXCJ 1314.5-2515. The remaining six clusters observed do not host extended emission of any kind.