The LOFAR long baseline snapshot calibrator survey^⋆

¹ ASTRON, the Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA, Dwingeloo, The Netherlands
e-mail: moldon@astron.nl
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
³ Jodrell Bank Center for Astrophysics, School of Physics and Astronomy, The University of Manchester, Manchester M13 9PL, UK
⁴ Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
⁵ Onsala Space Observatory, Dept. of Earth and Space Sciences, Chalmers University of Technology, 43992 Onsala, Sweden
⁶ Institute of Cosmology & Gravitation, University of Portsmouth, Burnaby Road, PO1 3 FX Portsmouth, UK
⁷ ARC Centre of Excellence for All-sky astrophysics (CAASTRO), Sydney Institute of Astronomy, University of Sydney, Australia
⁸ International Centre for Radio Astronomy Research – Curtin University, GPO Box U1987, WA 6845, Perth, Australia
⁹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
¹⁰ LESIA-Observatoire de Paris, CNRS, UPMC Univ Paris 6, Univ. Paris-Diderot, France
¹¹ Helmholtz-Zentrum Potsdam, DeutschesGeoForschungsZentrum GFZ, Department 1: Geodesy and Remote Sensing, Telegrafenberg, A17, 14473 Potsdam, Germany
¹² ShellTechnology Center, Bangalore, India
¹³ SRON Netherlands Insitute for Space Research, PO Box 800, 9700 AV Groningen, The Netherlands
¹⁴ Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The Netherlands
¹⁵ CSIRO Australia Telescope National Facility, PO Box 76, Epping NSW 1710, Australia
¹⁶ University of Twente, Drienerlolaon, 5, 7522 NB Enschede, The Netherlands
¹⁷ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, MA 02138, Cambridge, USA
¹⁸ Institute for Astronomy, University of Edinburgh, Royal Observatory of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
¹⁹ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
²⁰ Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, OX1 3RH, Oxford, UK
²¹ School of Physics and Astronomy, University of Southampton, Southampton, SO17 1BJ, UK
²² University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
²³ Research School of Astronomy and Astrophysics, Australian National University, Mt Stromlo Obs., via Cotter Road, A.C.T. 2611, Weston, Australia
²⁴ Anton Pannekoek Institute, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands
²⁵ Max Planck Institute for Astrophysics, Karl Schwarzschild Str. 1, 85741 Garching, Germany
²⁶ SmarterVision BV, Oostersingel 5, 9401 JX Assen, The Netherland
²⁷ Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
²⁸ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
²⁹ Laboratoire Lagrange, UMR7293, Université de Nice Sophia-Antipolis, CNRS, Observatoire de la Côte d’Azur, 06300 Nice, France
³⁰ LPC2E – Université d’Orléans/CNRS, France
³¹ Station de Radioastronomie de Nançay, Observatoire de Paris – CNRS/INSU, USR 704 - Univ. Orléans, OSUC, route de Souesmes, 18330 Nançay, France
³² Astronomisches Institut der Ruhr-Universität Bochum, Universitaetsstrasse 150, 44780 Bochum, Germany
³³ Astro Space Center of the Lebedev Physical Institute, Profsoyuznaya str. 84/32, 117997 Moscow, Russia
³⁴ Sodankylä Geophysical Observatory, University of Oulu, Tähteläntie 62, 99600 Sodankylä, Finland
³⁵ STFC Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, OX11 0 QX Didcot, UK
³⁶ Center for Information Technology (CIT), University of Groningen, The Netherlands
³⁷ Centre de Recherche Astrophysique de Lyon, Observatoire de Lyon, 9 av Charles André, 69561 Saint-Genis-Laval Cedex, France
³⁸ Fakultät für Physik, Universität Bielefeld, Postfach 100131, 33501, Bielefeld, Germany
³⁹ Department of Physics and Elelctronics, Rhodes University, PO Box 94, Grahamstown 6140, South Africa
⁴⁰ SKA South Africa, 3rd Floor, The Park, Park Road, Pinelands, 7405, South Africa
⁴¹ LESIA, UMR CNRS 8109, Observatoire de Paris, 92195 Meudon, France

Received: 23 September 2014
Accepted: 10 November 2014

Abstract

Aims. An efficient means of locating calibrator sources for international LOw Frequency ARray (LOFAR) is developed and used to determine the average density of usable calibrator sources on the sky for subarcsecond observations at 140 MHz.

Methods. We used the multi-beaming capability of LOFAR to conduct a fast and computationally inexpensive survey with the full international LOFAR array. Sources were preselected on the basis of 325 MHz arcminute-scale flux density using existing catalogues. By observing 30 different sources in each of the 12 sets of pointings per hour, we were able to inspect 630 sources in two hours to determine if they possess a sufficiently bright compact component to be usable as LOFAR delay calibrators.

Results. More than 40% of the observed sources are detected on multiple baselines between international stations and 86 are classified as satisfactory calibrators. We show that a flat low-frequency spectrum (from 74 to 325 MHz) is the best predictor of compactness at 140 MHz. We extrapolate from our sample to show that the sky density of calibrators that are sufficiently bright to calibrate dispersive and non-dispersive delays for the international LOFAR using existing methods is 1.0 per square degree.

Conclusions. The observed density of satisfactory delay calibrator sources means that observations with international LOFAR should be possible at virtually any point in the sky provided that a fast and efficient search, using the methodology described here, is conducted prior to the observation to identify the best calibrator.