2007A&A...467..943O

Spectral index images can be used to constraint the energy spectrum of relativistic electrons and magnetic field distribution in radio halos and relics, providing useful information to understand their formation, evolution and connection to cluster merger processes. We present low-frequency images of the two clusters of galaxies: A2744 and A2219, in which a wide diffuse emission is detected. Observations were made with the Very Large Array at the frequency of 325MHz. For both clusters deep Very Large Array 1.4GHz observations are available. Combining the 325MHz and 1.4GHz data, we obtained the spectral index images and the brightness radial profiles of the diffuse radio emission with a resolution of ∼1'. The azimuthally averaged spectral index in A2744 is constant to a value close to α≃1 up to a distance of 1 Mpc from the cluster center. However, the spectral index image shows the presence of localized regions in which the radio spectrum is significantly different from the average. The observed spectral index variations range from a minimum of α≃0.7±0.1 to a maximum α≃1.5±0.2. From the comparison of the spectral index with the X-ray data it is found for the first time that the flat spectrum regions of the radio halo tend to have higher temperature. In the case of A2219, the radio emission in the central regions of the cluster is dominated by a blend of discrete sources. The azimuthally averaged radio spectrum is α≃0.8 in the central region of the cluster and is close to α≃1 in the radio halo. The limited sensitivity of the 325MHz image does not allow us to detect all the radio halo structure seen at 1.4GHz and therefore no constraints on the point-to-point variations of the spectral index have been obtained for this cluster.