Diffuse polarized emission in the LOFAR Two-meter Sky Survey^★

¹ Dunlap Institute for Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, ON M5S 3H4, Canada
e-mail: cameron.van.eck@dunlap.utoronto.ca
² Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands
³ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁴ SUPA, Institute for Astronomy, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, UK
⁵ INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius (CA), Italy
⁶ Astronomical Observatory, Jagiellonian University, ul. Orla 171, 30-244 Kraków, Poland
⁷ Max Planck Institute for Astrophysics, Karl-Schwarzschild-Str. 1, 85748 Garching, Germany
⁸ Ludwig-Maximilians-Universität München, Geschwister-Scholl-Platz 1, 80539 München, Germany
⁹ Oxford e-Research Centre (OeRC), Keble Road, Oxford OX1 3QG, UK
¹⁰ IRAP, Université de Toulouse, CNRS, 9 avenue du Colonel Roche, BP 44346, 31028 Toulouse, Cedex 4, France
¹¹ CSIRO Astronomy and Space Science, PO Box 1130, Bentley, WA 6102, Australia
¹² ASTRON, the Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands
¹³ Ruđer Bošković Institute, Bijenička cesta 54, 10000 Zagreb, Croatia
¹⁴ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands

Received: 4 December 2018
Accepted: 25 January 2019

Abstract

Faraday tomography allows us to map diffuse polarized synchrotron emission from our Galaxy and use it to interpret the magnetic field in the interstellar medium (ISM). We have applied Faraday tomography to 60 observations from the LOFAR Two-meter Sky Survey (LOTSS) and produced a Faraday depth cube mosaic covering 568 square degrees at high Galactic latitudes, at 4.′3 angular resolution and 1 rad m⁻² Faraday depth resolution, with a typical noise level of 50–100 μJy per point spread function (PSF) per rotation measure spread function (RMSF; 40–80 mK RMSF⁻¹). While parts of the images are strongly affected by instrumental polarization, we observed diffuse polarized emission throughout most of the field, with typical brightness between 1 and 6 K RMSF⁻¹, and Faraday depths between − 7 and +25 rad m⁻². We observed many new polarization features, some up to 15° in length. These include two regions with very uniformly structured, linear gradients in the Faraday depth; we measured the steepness of these gradients as 2.6 and 13 rad m⁻² deg⁻¹. We also observed a relationship between one of the gradients and an H I filament in the local ISM. Other ISM tracers were also checked for correlations with our polarization data and none were found, but very little signal was seen in most tracers in this region. We conclude that the LOTSS data are very well suited for Faraday tomography, and that a full-scale survey with all the LOTSS data has the potential to reveal many new Galactic polarization features and map out diffuse Faraday depth structure across the entire northern hemisphere.