Deep Westerbork observations of Abell 2256 at 350 MHz

ASTRON, PO Box 2, 7990 AA Dwingeloo, The Netherlands e-mail: brentjens@astron.nl

Received: 26 January 2007
Accepted: 26 June 2008

Abstract

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 α = -1.61±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±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 μG. A value of 0.2 μG appears most reasonable given the currently available data.