The discovery of diffuse steep spectrum sources in Abell 2256
  Context: Hierarchical galaxy formation models indicate that during their
lifetime galaxy clusters undergo several mergers. Here we report on the
discovery of three diffuse radio sources in the periphery of Abell 2256, using
the GMRT.
  Aims: The aim of the observations was to search for diffuse ultra-steep
spectrum radio sources within the galaxy cluster Abell 2256.
  Methods: We have carried out GMRT 325 MHz radio continuum observations of
Abell 2256. V, R and I band images of the cluster were taken with the 4.2m WHT.
  Results: We have discovered three diffuse elongated radio sources located
about 1 Mpc from the cluster center. Two are located to the west of the cluster
center, and one to the southeast. The sources have a measured physical extent
of 170, 140 and 240 kpc, respectively. The two western sources are also visible
in deep low-resolution 115-165 MHz WSRT images, although they are blended into
a single source. For the combined emission of the blended source we find an
extreme spectral index of -2.05\pm 0.14 between 140 and 351 MHz. The extremely
steep spectral index suggests these two sources are most likely the result of
adiabatic compression of fossil radio plasma due to merger shocks.
  Conclusions: The discovery of the steep spectrum sources implies the
existence of a population of faint diffuse radio sources in (merging) clusters
with such steep spectra that they have gone unnoticed in higher frequency
(\gtrsim 1 GHz) observations. An exciting possibility therefore is that such
sources will determine the general appearance of clusters in low-frequency high
resolution radio maps as will be produced by for example LOFAR or LWA.
