Sub-arcsecond imaging with the International LOFAR Telescope

II. Completion of the LOFAR Long-Baseline Calibrator Survey^★

¹ University of Manchester, Jodrell Bank Centre for Astrophysics, Department of Physics & Astronomy, UK
e-mail: neal.jackson@manchester.ac.uk
² ICRAR, Curtin University, Bentley, WA 6102, Australia
³ Ventspils International Radio Astronomy Centre, Ventspils University College, Inzenieru 101, 3601, Ventspils, Latvia
⁴ Department of Physics, University of Durham, South Road, Durham DH1 3LE, UK
⁵ ASTRON, Hoogeveensedijk 4, 7990AA Dwingeloo, The Netherlands
⁶ Sterrewacht Leiden, Niels Bohrweg 2, 2333 CA, Leiden, The Netherlands
⁷ School of Physics, University College Dublin, Belfield, Dublin, Ireland
⁸ Swinburne University of Technology, Hawthorne, Victoria, Australia
⁹ Instituto de Astrofísica de Andalucía (IAA, CSIC), Glorieta de las Astronomía, s/n, 18008 Granada, Spain
¹⁰ Centre for Astrophysics Research, University of Hertfordshire, Hatfield AL10 9EU, UK
¹¹ CSIRO Astronomy and Space Science, PO Box 1130, Bentley WA 6102, Australia
¹² Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
¹³ Kapteyn Astronomical Institute, Groningen University, The Netherlands
¹⁴ Currently at Shell Technology Center, 562149, Bangalore, India
¹⁵ Science and Technology B.V., Delft, The Netherlands
¹⁶ Eindhoven University of Technology, De Rondom 70, 5612 AP Eindhoven, The Netherlands
¹⁷ DIFA - Universitá di Bologna, via Gobetti 93/2, 40129 Bologna, Italy
¹⁸ INAF - IRA, Via Gobetti 101, 40129 Bologna, Italy
¹⁹ IRA – INAF, via P. Gobetti 101, 40129 Bologna, Italy
²⁰ University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
²¹ Research School of Astronomy & Astrophysics, Mt. Stromlo Observatory, Cotter Road, Weston Creek, ACT 2611 Australia
²² Max-Planck Institute for Astrophysics, Karl-Schwarzschild-Straße 1, 85748 Garching, Germany
²³ Astrophysical Institute, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium
²⁴ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
²⁵ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029, Hamburg, Germany
²⁶ INAF - Istituto di Radioastronomia, via P. Gobetti 101, 40129, Bologna, Italy
²⁷ LPC2E - Université d’Orléans / CNRS, 45071 Orléans cedex 2, France
²⁸ Station de Radioastronomie de Nançay, Observatoire de Paris, PSL Research University, CNRS, Univ. Orléans, OSUC, 18330 Nançay, France
²⁹ Department of Physics, The George Washington University, Washington, DC 20052, USA
³⁰ Astronomy, Physics and Statistics Institute of Sciences (APSIS), The George Washington University, Washington, DC 20052, USA
³¹ Ruhr-University Bochum, Faculty of Physics and Astronomy, Astronomical Institute, 44780 Bochum, Germany
³² Space Radio-Diagnostics Research Centre, University of Warmia and Mazury, ul. Romana Prawochenskiego 9, 10-719 Olsztyn, Poland
³³ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
³⁴ ECAP. Friedrich-Alexander-University Erlangen-Nuremberg, Erwin-Rommel-Str. 1, 91058 Erlangen, Germany
³⁵ DESY, Platanenallee 6, 15738 Zeuthen, Germany
³⁶ Rijksuniversiteit Groningen, Nettelbosje 1, 9747AJ Groningen, The Netherlands
³⁷ Université Claude Bernard Lyon1, Ens de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon, 43 Boulevard du 11 Novembre 1918, 69100 Villeurbanne, France
³⁸ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³⁹ CBK PAN, Bartycka 18 A, 00-716 Warsaw, Poland
⁴⁰ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands
⁴¹ Fakultät für Physik, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany
⁴² Department of Physics and Electronics, Rhodes University, Makhanda (Grahamstown), South Africa
⁴³ South African Radio Astronomy Observatory, Cape Town, South Africa
⁴⁴ LESIA, Observatoire de Paris, CNRS, PSL, SU, UP, Place J. Janssen, 92190 Meudon, France

Received: 8 March 2021
Accepted: 29 April 2021

Abstract

The Low-Frequency Array (LOFAR) Long-Baseline Calibrator Survey (LBCS) was conducted between 2014 and 2019 in order to obtain a set of suitable calibrators for the LOFAR array. In this paper, we present the complete survey, building on the preliminary analysis published in 2016 which covered approximately half the survey area. The final catalogue consists of 30 006 observations of 24 713 sources in the northern sky, selected for a combination of high low-frequency radio flux density and flat spectral index using existing surveys (WENSS, NVSS, VLSS, and MSSS). Approximately one calibrator per square degree, suitable for calibration of ≥200 km baselines is identified by the detection of compact flux density, for declinations north of 30° and away from the Galactic plane, with a considerably lower density south of this point due to relative difficulty in selecting flat-spectrum candidate sources in this area of the sky. The catalogue contains indicators of degree of correlated flux on baselines between the Dutch core and each of the international stations, involving a maximum baseline length of nearly 2000 km, for all of the observations. Use of the VLBA calibrator list, together with statistical arguments by comparison with flux densities from lower-resolution catalogues, allow us to establish a rough flux density scale for the LBCS observations, so that LBCS statistics can be used to estimate compact flux densities on scales between 300 mas and 2′′, for sources observed in the survey. The survey is used to estimate the phase coherence time of the ionosphere for the LOFAR international baselines, with median phase coherence times of about 2 min varying by a few tens of percent between theshortest and longest baselines. The LBCS can be used to assess the structures of point sources in lower-resolution surveys, with significant reductions in the degree of coherence in these sources on scales between 2′′ and 300 mas. The LBCS survey sources show a greater incidence of compact flux density in quasars than in radio galaxies, consistent with unified schemes of radio sources. Comparison with samples of sources from interplanetary scintillation (IPS) studies with the Murchison Widefield Array shows consistent patterns of detection of compact structure in sources observed both interferometrically with LOFAR and using IPS.