The LOFAR Two-metre Sky Survey

V. Second data release^⋆

¹ ASTRON, the Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
e-mail: shimwell@astron.nl
² Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
³ Centre for Astrophysics Research, School of Physics, Astronomy and Mathematics, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
⁴ GEPI, Observatoire de Paris, Université PSL, CNRS, 5 place Jules Janssen, 92190 Meudon, France
⁵ Department of Physics & Electronics, Rhodes University, PO Box 94 Grahamstown 6140, South Africa
⁶ School of Physics and Astronomy, Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, EH9 3HJ Edinburgh, UK
⁷ Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
⁸ South African Radio Astronomy Observatory (SARAO), 2 Fir Street, Black River Business Park, Observatory, Cape Town, South Africa
⁹ Hamburger Sternwarte, University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
¹⁰ INAF-Istituto di Radioastronomia, via P. Gobetti 101, 40129 Bologna, Italy
¹¹ Astronomical Observatory, Jagiellonian University, ul. Orla 171, 30-244 Kraków, Poland
¹² Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
¹³ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010 6500 GL Nijmegen, The Netherlands
¹⁴ Jodrell Bank Centre for Astrophysics, Department of Physics and Astronomy, School of Natural Sciences, University of Manchester, Oxford Road, Manchester M13 9PL, UK
¹⁵ Centre for Extragalactic Astronomy, Department of Physics, Durham University, Durham DH1 3LE, UK
¹⁶ Institute for Computational Cosmology, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK
¹⁷ Kapteyn Astronomical Institute, University of Groningen, PO Box 800 9700 AV Groningen, The Netherlands
¹⁸ SURF, PO Box 94613 1090 GP Amsterdam, The Netherlands
¹⁹ Fakultät für Physik, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany
²⁰ School of Physical Sciences and Centre for Astrophysics & Relativity, Dublin City University, Glasnevin D09 W6Y4, Ireland
²¹ RAL Space, STFC Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, UK
²² School of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
²³ Institute of Cosmology and Gravitation, University of Portsmouth, Dennis Sciama Building, Burnaby Road, Portsmouth PO1 3FX, UK
²⁴ DIFA – Universitá di Bologna, via Gobetti 93/2, 40129 Bologna, Italy
²⁵ Center for Theoretical Physics, Polish Academy of Sciences, al. Lotników 32/46, 02-668 Warsaw, Poland
²⁶ Italian ALMA Regional Centre, Via Gobetti 101, 40129 Bologna, Italy
²⁷ INAF-Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
²⁸ European Southern Observatory, Karl-Schwarzchild-Strasse 2, 85748 Garching bei München, Germany
²⁹ Harvard-Smithsonian Center for Astrophysics, 60 Garden St, Cambridge, MA 02138, USA
³⁰ Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute, 44780 Bochum, Germany
³¹ Max Planck Institute for Astrophysics, Karl-Schwarzschild Str. 1, 85748 Garching, Germany
³² Ludwig-Maximilians-Universität München, Geschwister-Scholl-Platz 1, 80539 Munich, Germany
³³ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
³⁴ Space Radio-Diagnostics Research Centre, University of Warmia and Mazury, ul. Oczapowskiego 2, 10-719 Olsztyn, Poland
³⁵ CSIRO Space & Astronomy, PO Box 1130 Bentley, WA 6102, Australia
³⁶ Ruđer Bošković Institute, Bijenička cesta 54, 10000 Zagreb, Croatia
³⁷ Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, NCU, Grudziadzka 5, 87-100 Toruń, Poland
³⁸ INAF – IAPS, Via Fosso del Cavaliere 100, 00133 Roma, Italy
³⁹ National Centre for Nuclear Research, ul. Pasteura 7, 02-093 Warszawa, Poland
⁴⁰ Aix Marseille Univ. CNRS, CNES, LAM, Marseille, France
⁴¹ Dipartimento di Fisica, Università degli Studi di Torino, via Pietro Giuria 1, 10125 Torino, Italy
⁴² INAF-Osservatorio Astrofisico di Torino, via Osservatorio 20, 10025 Pino Torinese, Italy
⁴³ Consorzio Interuniversitario per la Fisica Spaziale, via Pietro Giuria 1, 10125 Torino, Italy
⁴⁴ Joint Institute for VLBI ERIC, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
⁴⁵ National Centre for Nuclear Research, Astrophysics Division, ul. Pasteura 7, 02-093 Warsaw, Poland
⁴⁶ School of Mathematics, Statistics, & Computer Science, University of KwaZulu-Natal, Durban, South Africa
⁴⁷ Astrophysics Research Centre, University of KwaZulu-Natal, Westville Campus, Durban 4000, South Africa
⁴⁸ Anton Pannekoek Institute, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands
⁴⁹ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
⁵⁰ Inter-University Institute for Data Intensive Astronomy (IDIA), Department of Physics and Astronomy, University of the Western Cape, Robert Sobukwe Road, 7535 Bellville, Cape Town, South Africa
⁵¹ Dunlap Institute for Astronomy and Astrophysics, University of Toronto, Toronto, ON M5S 3H4, Canada
⁵² SRON Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands
⁵³ National Astronomy Observatories, Chinese Academy of Science, Beijing 100101, PR China
⁵⁴ University of Chinese Academy of Sciences, Beijing 100049, PR China

Received: 19 October 2021
Accepted: 23 December 2021

Abstract

In this data release from the ongoing LOw-Frequency ARray (LOFAR) Two-metre Sky Survey we present 120–168 MHz images covering 27% of the northern sky. Our coverage is split into two regions centred at approximately 12h45m +44°30′ and 1h00m +28°00′ and spanning 4178 and 1457 square degrees respectively. The images were derived from 3451 h (7.6 PB) of LOFAR High Band Antenna data which were corrected for the direction-independent instrumental properties as well as direction-dependent ionospheric distortions during extensive, but fully automated, data processing. A catalogue of 4 396 228 radio sources is derived from our total intensity (Stokes I) maps, where the majority of these have never been detected at radio wavelengths before. At 6″ resolution, our full bandwidth Stokes I continuum maps with a central frequency of 144 MHz have: a median rms sensitivity of 83 μJy beam⁻¹; a flux density scale accuracy of approximately 10%; an astrometric accuracy of 0.2″; and we estimate the point-source completeness to be 90% at a peak brightness of 0.8 mJy beam⁻¹. By creating three 16 MHz bandwidth images across the band we are able to measure the in-band spectral index of many sources, albeit with an error on the derived spectral index of >  ± 0.2 which is a consequence of our flux-density scale accuracy and small fractional bandwidth. Our circular polarisation (Stokes V) 20″ resolution 120–168 MHz continuum images have a median rms sensitivity of 95 μJy beam⁻¹, and we estimate a Stokes I to Stokes V leakage of 0.056%. Our linear polarisation (Stokes Q and Stokes U) image cubes consist of 480 × 97.6 kHz wide planes and have a median rms sensitivity per plane of 10.8 mJy beam⁻¹ at 4′ and 2.2 mJy beam⁻¹ at 20″; we estimate the Stokes I to Stokes Q/U leakage to be approximately 0.2%. Here we characterise and publicly release our Stokes I, Q, U and V images in addition to the calibrated uv-data to facilitate the thorough scientific exploitation of this unique dataset.