Radio halos in nearby (z < 0.4) clusters of galaxies
  The Intra-Cluster Medium is characterized by thermal emission, and by the
presence of large scale magnetic fields. In some clusters of galaxies a diffuse
non-thermal emission is also present, located at the cluster center and named
radio halo. These sources indicate the existence of relativistic particles and
magnetic fields in the cluster volume. In this paper we collect data on all
known nearby cluster radio halos (z < 0.4), to discuss their statistical
properties and to investigate their origin. We searched for published data on
radio halos and reduced new and archive VLA data to increase the number of
known radio halos. We present data on 31 radio halos, 1 new relic source, and 1
giant filament. We note the discovery of a small size diffuse radio emission in
a cluster (A1213) with very low X-ray luminosity. Among statistical results we
confirm the correlation between the average halo radio spectral index and the
cluster temperature. We also discuss the high percentage of clusters where both
a relic and a radio halo is present. The sample of radio halos discussed here
represents the population of radio halos observable with present radio
telescopes. The new telescope generation is necessary for a more detailed
multifrequency study, and to investigate the possible existence of a population
of radio halos with different properties.
