Deep Westerbork observations of Abell 2256 at 350 MHz

TL;DR

This study presents deep polarimetric Westerbork observations of galaxy cluster Abell 2256 at 350 MHz, revealing detailed spectral and polarization properties of its radio sources, including a large halo and relics, with implications for magnetic fields.

Contribution

First detailed low-frequency polarimetric analysis of Abell 2256 revealing spectral breaks, polarization limits, and magnetic field estimates of cluster radio sources.

Findings

The halo has a diameter of about 1.2 Mpc with a spectral index of -1.61.

The ultra steep spectrum source shows a spectral break near 400 MHz, estimated to be at least 51 million years old.

No polarized flux detected in the cluster, with polarization fraction less than 1%. The magnetic field in relic filament G is estimated to be around 0.2 microGauss.

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 alpha=-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 uG. A value of 0.2 uG appears most reasonable given the currently available data.