Sub-arcsecond imaging with the International LOFAR Telescope: II. Completion of the LOFAR Long-Baseline Calibrator Survey

Jackson N, Badole S, Morgan J, Chhetri R, Prusis K, Nikolajevs A, Morabito L, Brentjens M, Sweijen F, Iacobelli M, Orru E, Sluman J, Blaauw R, Mulder H, Van Dijk P, Mooney S, Deller A, Moldon J, Callingham JR, Harwood J, Hardcastle M, Heald G, Drabent A, Mckean JP, Asgekar A, Avruch IM, Bentum MJ, Bonafede A, Brouw WN, Brueggen M, Butcher HR, Ciardi B, Coolen A, Corstanje A, Damstra S, Duscha S, Eisloeffel J, Falcke H, Garrett M, De Gasperin F, Griessmeier JM, Gunst AW, Van Haarlem MP, Hoeft M, Van Der Horst AJ, Juette E, Koopmans LVE, Krankowski A, Maat P, Mann G, Miley GK, Nelles A, Norden M, Paas M, Pandey VN, Pandey-Pommier M, Pizzo RF, Reich W, Rothkaehl H, Rowlinson A, Ruiter M, Shulevski A, Schwarz DJ, Smirnov O, Tagger M, Vocks C, Van Weeren RJ, Wijers R, Wucknitz O, Zarka P, Zensus JA, Zucca P (2022)

Publication Type: Journal article

Publication year: 2022

Journal

Book Volume: 658

Article Number: A2

DOI: 10.1051/0004-6361/202140756

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 the shortest 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 MurchisonWidefield Array shows consistent patterns of detection of compact structure in sources observed both interferometrically with LOFAR and using IPS.

Authors with CRIS profile

Involved external institutions

University of Manchester United Kingdom (GB) Curtin University Australia (AU) Astron Netherlands (NL) Leiden University Netherlands (NL) University College Dublin (UCD) Ireland (IE) Swinburne University of Technology Australia (AU) Spanish National Research Council / Consejo Superior de Investigaciones Científicas (CSIC) Spain (ES) University of Hertfordshire United Kingdom (GB) Commonwealth Scientific and Industrial Research Organisation (CSIRO) Australia (AU) Thüringer Landessternwarte Tautenburg (TLS) - Karl-Schwarzschild-Observatorium Germany (DE) University of Orléans / Université d'Orléans France (FR) Ventspils University College Latvia (LV) Durham University United Kingdom (GB) University of Bologna / Università di Bologna Italy (IT) University of Groningen / Rijksuniversiteit Groningen Netherlands (NL) Universität Hamburg (UHH) Germany (DE) Mount Stromlo Observatory Australia (AU) Max-Planck-Gesellschaft zur Förderung der Wissenschaften e.V. (MPG) / Max Planck Society for the Advancement of Science Germany (DE) Vrije Universiteit Brussel (VUB) Belgium (BE) Radboud University Nijmegen Netherlands (NL) George Washington University (GWU) United States (USA) (US) Ruhr-Universität Bochum (RUB) Germany (DE) University of Warmia and Mazury in Olsztyn Poland (PL) Leibniz Institute for Astrophysics Potsdam / Leibniz-Institut für Astrophysik Potsdam Germany (DE) Université Claude Bernard Lyon 1 (UCB) France (FR) Max-Planck-Institut für Radioastronomie / Max Planck Institute for Radio Astronomy Germany (DE) Space Research Centre / Centrum Badań Kosmicznych Poland (PL) Universität Bielefeld Germany (DE) Rhodes University South Africa (ZA) University of Amsterdam Netherlands (NL)

How to cite

APA:

Jackson, N., Badole, S., Morgan, J., Chhetri, R., Prusis, K., Nikolajevs, A.,... Zucca, P. (2022). Sub-arcsecond imaging with the International LOFAR Telescope: II. Completion of the LOFAR Long-Baseline Calibrator Survey. Astronomy & Astrophysics, 658. https://doi.org/10.1051/0004-6361/202140756

MLA:

Jackson, N., et al. "Sub-arcsecond imaging with the International LOFAR Telescope: II. Completion of the LOFAR Long-Baseline Calibrator Survey." Astronomy & Astrophysics 658 (2022).

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