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Polarization and Faraday rotation measure imaging at metre wavelengths with sub-arcsecond resolution: a foundational calibration strategy

Year: 2026
Type: article
Abstract: ABSTRACT Low-frequency radio polarimetric observations provide a powerful probe of magnetic fields in astrophysical sources and the intervening medium, as well as magnetospheric emission from compact objects such as pulsars, magnetically active stars, brown dwarfs, and planetary aurorae. With baselines of up to 2000 km, LOFAR offers a unique opportunity to study the low-frequency polarized Universe at sub-arcsecond resolution. However, polarimetric studies with LOFAR have so far been limited to angular resolutions of ${\sim }6\,\mathrm{ arcsec}$, resulting in stronger beam depolarization. Here, we present a calibration strategy that enables full-resolution polarimetric imaging with the LOFAR pan-European array. Our method applies full-Jones corrections to the international stations using an in-field unpolarized calibrator. In addition, when a sufficiently bright polarized source is present in the field, multi-epoch observations can be aligned in Faraday depth using a visibility-based correction that accounts for polarization angle and rotation measure offsets. This approach enables deeper combined imaging and deconvolution. We apply this strategy to the LOFAR ELAIS-N1 field, combining four 8-h observations for a total integration time of 32 h. At $0.3\,\mathrm{ arcsec}$ resolution, we detect two previously known polarized sources identified in lower-resolution studies, resolve additional polarized components, and localize emission regions with sub-arcsecond precision. We also identify a new polarized source and detect circularly polarized emission from the binary M-dwarf system CR Draconis, measuring its proper motion across epochs. These results demonstrate that sub-arcsecond polarimetry at metre wavelengths is now feasible with LOFAR, opening new science opportunities in the LOFAR2.0 era.
DOI: 10.1093/mnras/stag992
Authors: R J van Weeren, J M G H J de Jong, X K Le Saux, V A Chakawri, Q W E van Zegveld, D de Jong, S P O’Sullivan, F Sweijen, V H Mahatma, E De Rubeis, L K Morabito, D Alonso-López, A Bonafede, C Horellou, M van der Wild
Institutions: Leiden University, Leiden Observatory, Netherlands Institute for Radio Astronomy, Universidad Complutense de Madrid, Durham University, University of Cambridge, Universität Hamburg, Istituto di Radioastronomia di Bologna, University of Bologna, Chalmers University of Technology
Language: English
Open Access status: gold
Estimated APC paid: $3,046
Funders: California Department of Fish and Game, UK Research and Innovation, European Commission, Nederlandse Organisatie voor Wetenschappelijk Onderzoek, Institut sur la Nutrition et les Aliments Fonctionnels, National Institute of Corrections, Science and Technology Facilities Council, European Regional Development Fund, H2020 European Research Council
Awards: G1763825445, G285268981, G3148402578, Building Indo-UK collaborations towards the Square Kilometre Array, O.S.C.A.R.S. - Open Science Clusters’ Action for Research and Society, LOFAR-UK 2016-2019: LOFAR-UK Compute Facility, LOFAR-UK Request for Continued Funding, 2021-2024, and LOFAR2.0 Upgrade, Astronomy at Durham 2020-2023
OpenAlex ID: W7162553904
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