V-LoTSS: The circularly polarised LOFAR Two-metre Sky Survey^★

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
² ASTRON, Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
³ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
⁴ School of Physical Sciences and Centre for Astrophysics & Relativity, Dublin City University, Glasnevin D09 W6Y4, Ireland
⁵ Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
⁶ Centre for Astrophysics Research, Department of Physics, Astronomy and Mathematics, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
⁷ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands
⁸ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 5 Place Jules Janssen, 92195 Meudon, France
⁹ Aix-Marseille Université, CNRS, CNES, LAM, Marseille, France, 38 Rue Frédéric Joliot Curie, 13013 Marseille, France
¹⁰ School of Mathematics and Physics, The University of Queensland, St Lucia, QLD 4072, Australia
¹¹ Centre for Astrophysics, University of Southern Queensland, West Street, Toowoomba, QLD 4350, Australia
¹² Fakultät für Physik, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany
¹³ GEPI & ORN, Observatoire de Paris, Université PSL, CNRS, 5 Place Jules Janssen, 92190 Meudon, France
¹⁴ Department of Physics & Electronics, Rhodes University, PO Box 94, Grahamstown 6140, South Africa
¹⁵ School for Physical Sciences, The Open University, Milton Keynes MK11 9DL, England, UK
¹⁶ Space Science and Technology Division, RALSpace, STFC Rutherford Appleton Laboratory, Chilton, Didcot OX11 0NX, England, UK
¹⁷ ORN, Observatoire de Paris, CNRS, PSL, Université d’Orléans, Nançay, France
¹⁸ Ruhr University Bochum (RUB), Faculty of Physics and Astronomy, Astronomical Institute, Universitätsstr. 150, 44801 Bochum, Germany
¹⁹ Dipartimento di Fisica e Astronomia (DIFA), Universitá di Bologna, via Gobetti 93/2, 40129 Bologna, Italy
²⁰ Istituto Nazionale di Astrofisica (INAF) – Istituto di Radioastronomia (IRA), via Gobetti 101, 40129 Bologna, Italy
²¹ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029, Hamburg, Germany
²² Astronomical Observatory of the Jagiellonian University, ul. Orla 171, 30-244 Kraków, Poland
²³ Thüringer Landessternwarte, Sternwarte 5, D-07778 Tautenburg, Germany
²⁴ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
²⁵ CSIRO Space & Astronomy, PO Box 1130, Bentley, WA 6102, Australia
²⁶ Space Radio-Diagnostics Research Centre, University of Warmia and Mazury, ul.Oczapowskiego 2, 10-719 Olsztyn, Poland
²⁷ Institut für Theoretische Physik und Astrophysik, Universität Würzburg, Emil-Fischer-Straße 31, 97074 Würzburg, Germany
²⁸ Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, NCU, Grudziadzka 5, 87-100 Toruń, Poland
²⁹ School of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
³⁰ Centre for Extragalactic Astronomy, Department of Physics, Durham University, Durham DH1 3LE, UK
³¹ Institute for Computational Cosmology, Department of Physics, University of Durham, South Road, Durham DH1 3LE, UK
³² Astronomy Department, Universidad de Concepción, Casilla 160-C, Concepción, Chile
³³ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
³⁴ Facultad de Ciencias, Universidad de Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain
³⁵ School of Sciences, European University Cyprus, Diogenes street, Engomi, 1516 Nicosia, Cyprus
³⁶ Astronomy and Astrophysics Section, Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin D02 XF86, Ireland
³⁷ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
³⁸ Inter-University Institute for Data Intensive Astronomy, Department of Astronomy, University of Cape Town, 7701 Rondebosch, Cape Town, South Africa
³⁹ Inter-University Institute for Data Intensive Astronomy, Department of Physics and Astronomy, University of the Western Cape, 7535 Bellville, Cape Town, South Africa

Received: 28 November 2022
Accepted: 17 December 2022

Abstract

We present the detection of 68 sources from the most sensitive radio survey in circular polarisation conducted to date. We used the second data release of the 144 MHz LOFAR Two-metre Sky Survey to produce circularly polarised maps with a median noise of 140 µJy beam⁻¹ and resolution of 20″ for ≈27% of the northern sky (5634 deg²). The leakage of total intensity into circular polarisation is measured to be ≈0.06%, and our survey is complete at flux densities ≥1 mJy. A detection is considered reliable when the circularly polarised fraction exceeds 1%. We find the population of circularly polarised sources is composed of four distinct classes: stellar systems, pulsars, active galactic nuclei, and sources unidentified in the literature. The stellar systems can be further separated into chromospherically active stars, M dwarfs, and brown dwarfs. Based on the circularly polarised fraction and lack of an optical counterpart, we show it is possible to infer whether the unidentified sources are likely unknown pulsars or brown dwarfs. By the completion of this survey of the northern sky, we expect to detect 300±100 circularly polarised sources.