Sub-arcsecond imaging with the International LOFAR Telescope

I. Foundational calibration strategy and pipeline

¹ Centre for Extragalactic Astronomy, Department of Physics, Durham University, Durham DH1 3LE, UK
e-mail: leah.k.morabito@durham.ac.uk
² Institute for Computational Cosmology, Department of Physics, University of Durham, South Road, Durham DH1 3LE, UK
³ Jodrell Bank Centre for Astrophysics, Department of Physics and Astronomy, School of Natural Sciences, University of Manchester, Manchester M13 9PL, UK
⁴ School of Physics, University College Dublin, Belfield, Dublin 4, Ireland
⁵ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁶ Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV Groningen, The Netherlands
⁷ ASTRON, Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 PD, Dwingeloo, The Netherlands
⁸ Department of Astronomy, AlbaNova University Center, Stockholm University, 10691 Stockholm, Sweden
⁹ Institut für Theoretische Physik und Astrophysik, Universität Würzburg, Emil-Fischer-Str. 31, 97074 Würzburg, Germany
¹⁰ Universita di Bologna, Via Zamboni, 33, 40126 Bologna BO, Italy
¹¹ Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
¹² School of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK
¹³ Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
¹⁴ INAF – Istituto di Radioastronomia, Via P. Gobetti 101, 40129, Bologna, Italy
¹⁵ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
¹⁶ Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Mail H30, PO Box 218, Hawthorn, VIC 3122, Australia
¹⁷ Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
¹⁸ Instituto de Astrofísica de Andalucía (IAA, CSIC), Glorieta de las Astronomía, s/n, 18008 Granada, Spain
¹⁹ GEPI & USN, Observatoire de Paris, CNRS, Université Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France
²⁰ Centre for Radio Astronomy Techniques and Technologies, Department of Physics and Electronics, Rhodes University, Grahamstown 6140, South Africa
²¹ Technische Universität Berlin, Institut für Geodäsie und Geoinformationstechnik, Fakultẗ VI, Sekr. KAI 2-2, Kaiserin-Augusta-Allee 104-106, 10553 Berlin, Germany
²² GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany
²³ Currently at Shell Technology Center, Bangalore 562149, India
²⁴ Science and Technology B.V., Delft, The Netherlands
²⁵ Joint Institute for VLBI ERIC (JIVE), Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
²⁶ Eindhoven University of Technology, De Rondom 70, 5612 AP Eindhoven, The Netherlands
²⁷ DIFA – Universitá di Bologna, via Gobetti 93/2, 40129 Bologna, Italy
²⁸ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
²⁹ Research School of Astronomy & Astrophysics, Mt. Stromlo Observatory, Cotter Road, Weston Creek, ACT 2611, Australia
³⁰ Max-Planck Institute for Astrophysics, Karl-Schwarzschild-Strasse 1, 85748 Garching, Germany
³¹ Astrophysical Institute, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium
³² Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
³³ LPC2E – Université d’Orléans/CNRS, 45071 Orléans cedex 2, France
³⁴ Station de Radioastronomie de Nançay, Observatoire de Paris, PSL Research University, CNRS, Univ. Orléans, OSUC, 18330 Nançay, France
³⁵ Department of Physics, The George Washington University, 725 21st Street NW, Washington, DC 20052, USA
³⁶ Astronomy, Physics and Statistics Institute of Sciences (APSIS), The George Washington University, Washington, DC 20052, USA
³⁷ Ruhr University Bochum, Faculty of Physics and Astronomy, Astronomical Institute, 44780 Bochum, Germany
³⁸ Space Radio-Diagnostics Research Centre, University of Warmia and Mazury, ul. Romana Prawochenskiego 9, 10-719 Olsztyn, Poland
³⁹ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁴⁰ ECAP, Friedrich-Alexander-University Erlangen-Nuremberg, Erwin-Rommel-Str. 1, 91058 Erlangen, Germany
⁴¹ DESY, Platanenallee 6, 15738 Zeuthen, Germany
⁴² SURF/SURFsara, PO Box 94613, 1090 GP Amsterdam, The Netherlands
⁴³ CIT, Rijksuniversiteit Groningen, Nettelbosje 1, 9747AJ Groningen, The Netherlands
⁴⁴ Université Claude Bernard Lyon1, Ens de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon, 43 Boulevard du 11 Novembre 1918, 69100 Villeurbanne, France
⁴⁵ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁴⁶ CBK PAN, Bartycka 18 A, 00-716 Warsaw, Poland
⁴⁷ Fakultät für Physik, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany
⁴⁸ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands
⁴⁹ Astronomical Observatory of the Jagiellonian University, ul. Orla171, 30-244 Kraków, Poland
⁵⁰ LESIA, Observatoire de Paris, CNRS, PSL, SU, UP, Place J. Janssen, 92190 Meudon, France

Received: 24 February 2021
Accepted: 1 April 2021

Abstract

The International LOFAR Telescope is an interferometer with stations spread across Europe. With baselines of up to ~2000 km, LOFAR has the unique capability of achieving sub-arcsecond resolution at frequencies below 200 MHz. However, it is technically and logistically challenging to process LOFAR data at this resolution. To date only a handful of publications have exploited this capability. Here we present a calibration strategy that builds on previous high-resolution work with LOFAR. It is implemented in a pipeline using mostly dedicated LOFAR software tools and the same processing framework as the LOFAR Two-metre Sky Survey (LoTSS). We give an overview of the calibration strategy and discuss the special challenges inherent to enacting high-resolution imaging with LOFAR, and describe the pipeline, which is publicly available, in detail. We demonstrate the calibration strategy by using the pipeline on P205+55, a typical LoTSS pointing with an 8 h observation and 13 international stations. We perform in-field delay calibration, solution referencing to other calibrators in the field, self-calibration of these calibrators, and imaging of example directions of interest in the field. We find that for this specific field and these ionospheric conditions, dispersive delay solutions can be transferred between calibrators up to ~1.5° away, while phase solution transferral works well over ~1°. We also demonstrate a check of the astrometry and flux density scale with the in-field delay calibrator source. Imaging in 17 directions, we find the restoring beam is typically ~0.3′′ ×0.2′′ although this varies slightly over the entire 5 deg² field of view. We find we can achieve ~80–300 μJy bm⁻¹ image rms noise, which is dependent on the distance from the phase centre; typical values are ~90 μJy bm⁻¹ for the 8 h observation with 48 MHz of bandwidth. Seventy percent of processed sources are detected, and from this we estimate that we should be able to image roughly 900 sources per LoTSS pointing. This equates to ~ 3 million sources in the northern sky, which LoTSS will entirely cover in the next several years. Future optimisation of the calibration strategy for efficient post-processing of LoTSS at high resolution makes this estimate a lower limit.