  Several arguments have been presented in the literature to support the
connection between radio halos and cluster mergers. The spectral index
distributions of the halos in A665 and A2163 provide a new strong confirmation
of this connection, i.e. of the fact that the cluster merger plays an important
role in the energy supply to the radio halos. Features of the spectral index
(flattening and patches) are indication of a complex shape of the radiating
electron spectrum, and are therefore in support of electron reacceleration
models. Regions of flatter spectrum are found to be related to the recent
merger. In the undisturbed cluster regions, instead, the spectrum steepens with
the distance from the cluster center.The plot of the integrated spectral index
of a sample of halos versus the cluster temperature indicates that clusters at
higher temperature tend to host halos with flatter spectra. This correlation
provides further evidence of the connection between radio emission and cluster
mergers.
