The intracluster magnetic field power spectrum in Abell 2255

Aims. The goal of this work is to constrain the strength and structure of the magnetic field in the nearby cluster of galaxies A2255. At radio wavelengths A2255 is characterized by the presence of a polarized radio halo at the cluster center, a relic source at the cluster periphery, and several embedded radio galaxies. The polarized radio emission from all these sources is modified by Faraday rotation as it traverses the magnetized intra-cluster medium. The distribution of Faraday rotation can be used to probe the magnetic field strength and topology in the cluster. Methods. For this purpose, we performed Very Large Array observations at 3.6 and 6 cm of four polarized radio galaxies embedded in A2255, obtaining detailed rotation measure images for three of them. We analyzed these data together with the very deep radio halo image recently obtained by us. We simulated random 3-dimensional magnetic field models characterized by different power spectra and produced synthetic rotation measure and radio halo images. By comparing the simulations with the data we are able to determine the strength and the power spectrum of the intra-cluster magnetic field fluctuations which best reproduce the observations. Results. The data require a steepening of the power spectrum spectral index from n = 2, at the cluster center, up to n = 4, at the cluster periphery and the presence of filamentary structures on large scales. The average magnetic field strength at the cluster center is 2.5 mu G. The field strength declines from the cluster center outward with an average magnetic field strength calculated over 1 Mpc(3) of similar to 1.2 mu G.