Rotation measure synthesis of galactic polarized emission with the DRAO 26-m telescope

Abstract

Radio polarimetry at decimeter wavelengths is the principal source of information on the Galactic magnetic field. The diffuse polarized emission is strongly influenced by Faraday rotation in the magneto-ionic medium and rotation measure (RM) is the prime quantity of interest, implying that all Stokes parameters must be measured over wide frequency bands with many frequency channels. The Dominion Radio Astrophysical Observatory (DRAO) 26 m Telescope has been equipped with a wide-band feed, a polarization transducer to deliver both hands of circular polarization, and a receiver, all operating from 1277 to 1762MHz. Half-power beamwidth is between 40 and 30arcmin. A digital Field-Programmable Gate Array spectrometer, based on commercially available components, produces all Stokes parameters in 2048 frequency channels over a 485-MHz bandwidth. Signals are digitized to 8 bits and a Fast Fourier Transform is applied to each data stream. Stokes parameters are then generated in each frequency channel. This instrument is in use at DRAO for a Northern sky polarization survey. Observations consist of scans up and down the Meridian at a drive rate of ∼0°.9 per minute to give complete coverage of the sky between declinations -30° and 90°. This paper presents a complete description of the receiver and data acquisition system. Only a small fraction of the frequency band of operation is allocated for radio astronomy, and about 20% of the data are lost to interference. The first 8% of data from the survey are used for a proof-of-concept study, which has led to the first application of RM-Synthesis to the diffuse Galactic emission obtained with a single-antenna telescope. We find RM values for the diffuse emission as high as ∼ ± 100radm-2, much higher than recorded in earlier work. © 2010. The American Astronomical Society.