A search for steep spectrum radio relics and halos with the GMRT
  Context: Diffuse radio emission, in the form of radio halos and relics,
traces regions in clusters with shocks or turbulence, probably produced by
cluster mergers. Some models of diffuse radio emission in clusters indicate
that virtually all clusters should contain diffuse radio sources with a steep
spectrum. External accretion shocks associated with filamentary structures of
galaxies could also accelerate electrons to relativistic energies and hence
produce diffuse synchrotron emitting regions. Here we report on Giant Metrewave
Radio Telescope (GMRT) observations of a sample of steep spectrum sources from
the 74 MHz VLSS survey. These sources are diffuse and not associated with
nearby galaxies.
  Aims: The main aim of the observations is to search for diffuse radio
emission associated with galaxy clusters or the cosmic web.
  Methods: We carried out GMRT 610 MHz continuum observations of unidentified
diffuse steep spectrum sources.
  Results: We have constructed a sample of diffuse steep spectrum sources,
selected from the 74 MHz VLSS survey. We identified eight diffuse radio sources
probably all located in clusters. We found five radio relics, one cluster with
a giant radio halo and a radio relic, and one radio mini-halo. By complementing
our observations with measurements from the literature we find correlations
between the physical size of relics and the spectral index, in the sense that
smaller relics have steeper spectra. Furthermore, larger relics are mostly
located in the outskirts of clusters while smaller relics are located closer to
the cluster center.
