Untangling cosmic magnetic fields: Faraday tomography at metre wavelengths with LOFAR
Journal article, 2018

The technique of Faraday tomography is a key tool for the study ofmagnetised plasmas in the new era of broadband radio-polarisation observations. In particular, observations at metre wavelengths provide significantly better Faraday depth accuracies compared to traditional centimetre-wavelength observations. However, the effect of Faraday depolarisationmakes the polarised signal very challenging to detect at metre wavelengths (MHz frequencies). In this work, Faraday tomography is used to characterise the Faraday rotation properties of polarised sources found in data from the LOFAR Two-Metre Sky Survey (LoTSS). Of the 76 extragalactic polarised sources analysed here, we find that all host a radio-loud AGN (Active Galactic Nucleus). The majority of the sources (~64%) are large FRII radio galaxies with a median projected linear size of 710 kpc and median radio luminosity at 144 MHz of 4 × 1026 W Hz-1 (with ~13% of all sources having a linear size > 1 Mpc). In several cases, both hotspots are detected in polarisation at an angular resolution of ~20'. One such case allowed a study of intergalactic magnetic fields on scales of 3.4 Mpc. Other detected source types include an FRI radio galaxy and at least eight blazars. Most sources display simple Faraday spectra, but we highlight one blazar that displays a complex Faraday spectrum, with two close peaks in the Faraday dispersion function.

Large-scale structure

Faraday tomography

Magnetic fields


Milky Way


Shane P. O'Sullivan

Universität Hamburg

M. Brueggen

Universität Hamburg

C.L. Van Eck

University of Toronto

M. J. Hardcastle

University of Hertfordshire

M. Haverkorn

Radboud University

T. W. Shimwell

Netherlands Institute for Radio Astronomy (ASTRON)

C. Tasse

GEPI - Galaxies, Etoiles, Physique, Instrumentation

V. Vacca

INAF Osservatorio Astronomico di Cagliari

Cathy Horellou

Chalmers, Space, Earth and Environment, Onsala Space Observatory

G. Heald

CSIRO Astronomy and Space Science


20754434 (eISSN)

Vol. 6 4 126

Subject Categories

Astronomy, Astrophysics and Cosmology

Signal Processing

Fusion, Plasma and Space Physics



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