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We performed GMRT low frequency observations of the radio halos, relics andnew candidates belonging to the GMRT Radio Halo Cluster Sample first observedat 610 MHz. High sensitivity imaging was performed using the GMRT at 325 MHzand 240 MHz. The properties of the diffuse emission in each cluster werecompared to our 610 MHz images and/or literature information available at otherfrequencies, in order to derive the integrated spectra over a wide frequencyrange.Beyond the classical radio halos, whose spectral index $\alpha$ is in therange $\sim1.2\div1.3$ (S$\propto\nu^{-\alpha}$), we found sources with$\alpha\sim1.6\div1.9$. This result supports the idea that the spectra of theradiating particles in radio halos is not universal, and that inefficientmechanisms of particle acceleration are responsible for their origin. We alsofound a variety of brightness distributions, i.e. centrally peaked as well asclumpy halos. Even though the thermal and relativistic plasma tend to occupythe same cluster volume, in some cases a positional shift between the radio andX-ray peaks of emission is evident. Our observations also revealed theexistence of diffuse cluster sources which cannot be easily classified eitheras halos or relics. New candidate relics were found in A1300 and in A1682, andin some clusters "bridges" of radio emission have been detected, connecting therelic and radio halo emission. Combining our new data with literatureinformation, we derived the LogL$_{\rm X}$-LogP$_{\rm 325 MHz}$ correlation forradio halos, and investigated the possible trend of the spectral index of radiohalos with the temperature of the intracluster medium.

Low frequency follow up of radio halos and relics in the GMRT Radio Halo Cluster Survey