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