Abstract
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 centimetre-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 (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 × 10 26 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.
Highlights
Advances in radio-receiver technology that enable wide-spanning observations, continuous frequency ranges, at a high spectral resolution, have opened a new parameter space for studiesGalaxies 2018, 6, 126; doi:10.3390/galaxies6040126 www.mdpi.com/journal/galaxiesGalaxies 2018, 6, 126 of cosmic magnetism
It is the properties of these 76 sources that we present in this paper. This is only ∼0.4% of the 19,233 radio sources with total flux densities greater than 10 mJy in the LOFAR Two-Metre Sky Survey (LoTSS)-DR1 area. This emphasises the scarcity of polarised sources at 144 MHz, with a polarised source sky density of ∼0.18 per square degree1
The most striking feature of the polarised sources in the LoTSS catalog is that the majority are large FRII radio galaxies, with a median linear size of 710 kpc
Summary
Galaxies 2018, 6, 126 of cosmic magnetism They bring the technique of Faraday tomography to the fore, which allows detailed studies of the distribution of linearly polarised radiation as a function of Faraday depth, F (φ), which probes the physical properties of magnetised plasma along the line of sight. In this formalism, complex linearly polarised intensity P(λ2 ) is expressed as: P ( λ2 ) = Z ∞ −∞ F (φ) e2iφλ dφ, (1). The distribution of Faraday depths within the synthesised beam of the telescope can cause Faraday depolarisation, which makes the polarised signal fainter and more difficult to detect at long wavelengths [9]
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