The LOFAR LBA Sky Survey

I. Survey description and preliminary data release^⋆

¹ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
e-mail: fdg@hs.uni-hamburg.de
² INAF – Istituto di Radioastronomia, via P. Gobetti 101, 40129 Bologna, Italy
³ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁴ Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
⁵ ASTRON, the Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands
⁶ Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
⁷ Ruhr-Universität Bochum, Universitätsstr 150/NA7, 44801 Bochum, Germany
⁸ DIFA – Universitá di Bologna, via Gobetti 93/2, 40129 Bologna, Italy
⁹ Astronomical Observatory, Jagiellonian University, ul. Orla 171, 30-244 Kraków, Poland
¹⁰ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475, USA
¹¹ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands
¹² CSIRO Astronomy and Space Science, PO Box 1130, Bentley, WA 6102, Australia
¹³ Institut für Theoretische Physik und Astrophysik, Universität Würzburg, Emil-Fischer-Str. 31, 97074 Würzburg, Germany
¹⁴ National Centre for Nuclear Research, ul. Pasteura 7, 02-093 Warsaw, Poland
¹⁵ School of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
¹⁶ Centre for Extragalactic Astronomy, Department of Physics, Durham University, Durham DH1 3LE, UK
¹⁷ Institute for Computational Cosmology, Department of Physics, University of Durham, South Road, Durham DH1 3LE, UK
¹⁸ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
¹⁹ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands
²⁰ GEPI&USN, Observatoire de Paris, CNRS, Université Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France
²¹ Centre for Radio Astronomy Techniques and Technologies, Rhodes University, Grahamstown 6140, South Africa
²² Dep. of Physics & Astronomy, University of the Western Cape, Robert Sobukwe Road, 7535 Bellville, Cape Town, South Africa
²³ LESIA, UMR CNRS 8109, Observatoire de Paris, 92195 Meudon, France

Received: 11 January 2021
Accepted: 8 February 2021

Abstract

Context. The LOw Frequency ARray (LOFAR) is the only radio telescope that is presently capable of high-sensitivity, high-resolution (i.e. < 1 mJy beam⁻¹ and < 15″) observations at ultra-low frequencies (< 100 MHz). To utilise these capabilities, the LOFAR Surveys Key Science Project is undertaking a large survey to cover the entire northern sky with Low Band Antenna (LBA) observations.

Aims. The LOFAR LBA Sky Survey (LoLSS) aims to cover the entire northern sky with 3170 pointings in the frequency range between 42 − 66 MHz, at a resolution of 15″ and at a sensitivity of 1 mJy beam⁻¹ (1σ). In this work, we outline the survey strategy, the observational status, and the calibration techniques. We also briefly describe several of our scientific motivations and present the preliminary public data release.

Methods. The preliminary images were produced using a fully automated pipeline aimed at correcting all direction-independent effects in the data. Whilst the direction-dependent effects, such as those from the ionosphere, have not yet been corrected, the images presented in this work are still ten times more sensitive than previous available surveys at these low frequencies.

Results. The preliminary data release covers 740 deg² around the HETDEX spring field region at an angular resolution of 47″ with a median noise level of 5 mJy beam⁻¹. The images and the catalogue of 25 247 sources have been publicly released. We demonstrate that the system is capable of reaching a root mean square (rms) noise of 1 mJy beam⁻¹ and an angular resolution of 15″ once direction-dependent effects are accounted for.

Conclusions. LoLSS will provide the ultra-low-frequency information for hundreds of thousands of radio sources, providing critical spectral information and producing a unique data set that can be used for a wide range of science topics, such as the search for high redshift galaxies and quasars, the study of the magnetosphere of exoplanets, and the detection of the oldest populations of cosmic-rays in galaxies, clusters of galaxies, as well as those produced by active galactic nuclei.