Untangling cosmic magnetic fields: Faraday tomography at metre
  wavelengths with LOFAR
  The technique of Faraday tomography is a key tool for the study of magnetised
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 cm-wavelength observations.
However, the effect of Faraday depolarisation makes 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. The majority of the sources ($\sim$64%) are large FRII radio
galaxies with a median projected linear size of 710 kpc and median radio
luminosity at 144 MHz of $4\times10^{26}$ W Hz$^{-1}$ (with $\sim$13% of all
sources having a linear size $>1$ Mpc). In several cases, both hotspots are
detected in polarisation at an angular resolution of $\sim$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 8 blazars. Most
sources display simple Faraday spectra, however, we highlight one blazar that
displays a complex Faraday spectrum, with two close peaks in the Faraday
dispersion function.
