The Coma cluster magnetic field from Faraday rotation measures

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 eta, where eta 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 similar to 2 kpc and maximum scale similar to 34 kpc. The central magnetic field strength and radial slope are constrained to be in the range (B-0 = 3.9 mu G; eta = 0.4) and (B-0 = 5.4 mu G; eta = 0.7) within 1 sigma. The best agreement between observations and simulations is achieved for B-0 = 4.7 mu G; eta = 0.5. Values of B-0 > 7 mu G and <3 mu G as well as eta < 0.2 and eta > 1.0 are incompatible with RM data at 99% confidence level.