Non thermal emission from galaxy clusters: radio halos
  The number of diffuse radio halos in clusters of galaxies has grown in recent years, making it possible to derive statistical properties of these sources and of the hosting clusters. We show that diffuse sources are associated with X-ray luminous clusters which have undergone recent merger processes. The radio and X-ray structures are often similar, and correlations are found between radio and X-ray parameters. A model for the formation and maintenance of these sources is suggested, including a first phase of relativistic particle injection by past major merger events, starbust and AGN activity and a second phase of reacceleration by the energy supplied from shocks and turbulence in a recent merger event. 
L. Feretti (1), G. Brunetti (1,2), G. Giovannini (1,3), F. Govoni   (1,2), G. Setti(1,2)((1) IRA Bologna Italy (2) Dip. Astr...