Radio continuum observations of new radio halos and relics from the NVSS
  and WENSS surveys
  Radio halos and relics are diffuse radio sources found in galaxy clusters
showing significant substructure at X-ray wavelengths. These sources provide
important information about non-thermal processes taking place in the ICM.
Until now only a few dozen relics and halos are known, while models predict
that a much larger number of these sources exist. In this paper we present the
results of an extensive observing campaign to search for new diffuse radio
sources in galaxy clusters. We discovered 6 new radio relics, including a
probable double relic system, and 2 radio halos. In addition, we confirm the
presence of diffuse radio emission in four galaxy clusters. By constructing a
sample of 35 radio relics we find that relics are mostly found along the major
axis of the X-ray emission from the ICM, while their orientation is
perpendicular to this axis. This is consistent with the scenario that relics
trace merger shock waves. The major and minor axis ratio distribution of the
ICM for clusters with relics is also broader than that of the NORAS-REFLEX
sample. We compared the X-ray luminosity and redshift distributions of clusters
with relics to an X-ray selected sample from the NORAS and REFLEX surveys. We
find tentative evidence for an increase of the cluster's relic fraction with
X-ray luminosity and redshift. [abridged]
