Deep polarimetric Westerbork observations of the galaxy cluster Abell~2256 are presented , covering a frequency range of 325–377 MHz .
The central halo source has a diameter of the order of 1.2 Mpc ( 18′ ) , which is somewhat larger than at 1.4 GHz .
With \alpha = -1.61 \pm 0.04 , the halo spectrum between 1.4 GHz and 22.25 MHz is less steep than previously thought .
The centre of the ultra steep spectrum source in the eastern part of the cluster exhibits a spectral break near 400 MHz .
It is estimated to be at least 51 million years old , but possibly older than 125 million years .
A final measurement requires observations in the 10–150 MHz range .
It remains uncertain whether the source is a radio tail of Fabricant galaxy 122 , situated in the northeastern tip of the source .
Faraday rotation measure synthesis revealed no polarized flux at all in the cluster .
The polarization fraction of the brightest parts of the relic area is less than 1 % .
The RM-synthesis nevertheless revealed 9 polarized sources in the field enabling an accurate measurement of the Galactic Faraday rotation ( -33 \pm 2 rad m ^ { -2 } in front of the relic ) .
Based on its depolarization on longer wavelengths , the line-of-sight magnetic field in relic filament G is estimated to be between 0.02 and 2 \mu G. A value of 0.2 \mu G appears most reasonable given the currently available data .