LBCS: The LOFAR Long-Baseline Calibrator Survey

¹ University of Manchester, School of Physics and Astronomy, Jodrell Bank Centre for Astrophysics, Oxford Road, Manchester M13 9PL, UK
² ASTRON, Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands
³ Department of Earth & Space Sciences, Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
⁴ Sterrewacht Leiden, University of Leiden, 2300 RA Leiden, The Netherlands
⁵ Max-Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁶ Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
⁷ University of Portsmouth, Winston Churchill Ave, Portsmouth PO1 2UP, UK
⁸ International Centre for Radio Astronomy Research (ICRAR), University of Western Australia, Crawley WA 6009, Australia
⁹ CAASTRO, Arc Centre of Excellence for All-Sky Astrophysics, 2006 Australia
¹⁰ Helmholtz-Zentrum Potsdam, DeutschesGeoForschungsZentrum GFZ, Department 1: Geodesy and Remote Sensing, Telegrafenberg, A17, 14473 Potsdam, Germany
¹¹ SRON Netherlands Insitute for Space Research, PO Box 800, 9700 AV Groningen, The Netherlands
¹² Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The Netherlands
¹³ CSIRO Astronomy and Space Science, 26 Dick Perry Avenue, Kensington, WA 6151, Australia
¹⁴ Joint Institute for VLBI in Europe, Dwingeloo, Postbus 2, 7990 AA Dwingeloo, The Netherlands
¹⁵ University of Twente, 7522 NBEnschede, The Netherlands
¹⁶ Institute for Astronomy, University of Edinburgh, Royal Observatory of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
¹⁷ University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
¹⁸ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
¹⁹ Max Planck Institute for Astrophysics, Karl Schwarzschild Str. 1, 85741 Garching, Germany
²⁰ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
²¹ SmarterVision BV, Oostersingel 5, 9401 JX Assen, The Netherlands
²² Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany
²³ LPC2E – Université d’Orléans/CNRS, 450751 Orléans, Cedex 2, France
²⁴ Station de Radioastronomie de Nançay, Observatoire de Paris – CNRS/INSU, USR 704 – Univ. Orléans, OSUC, route de Souesmes, 18330 Nançay, France
²⁵ National Radio Astronomy Observatory, 1003 Lopezville Road, Socorro, NM 87801-0387, USA
²⁶ Astronomisches Institut der Ruhr-Universität Bochum, Universitaetsstrasse 150, 44780 Bochum, Germany
²⁷ National Astronomical Observatory of Japan, Tokyo, Japan
²⁸ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
²⁹ Sodankylä Geophysical Observatory, University of Oulu, Tähteläntie 62, 99600 Sodankylä, Finland
³⁰ STFC Rutherford Appleton Laboratory, Harwell Science and Innovation Campus, Didcot OX11 0QX, UK
³¹ Radboud University Radio Lab, Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
³² Centre de Recherche Astrophysique de Lyon, Observatoire de Lyon, 9 Av. Charles André, 69561 Saint Genis Laval Cedex, France
³³ Fakultät für Physik, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany
³⁴ Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA
³⁵ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
³⁶ LESIA & USN, Observatoire de Paris, CNRS, PSL/SU/UPMC/ UPD/SPC, Place J. Janssen, 92195 Meudon, France

^⋆ Corresponding author: Neal Jackson, e-mail: neal.jackson@manchester.ac.uk

Received: 27 May 2016
Accepted: 22 July 2016

Abstract

We outline the LOFAR Long-Baseline Calibrator Survey (LBCS), whose aim is to identify sources suitable for calibrating the highest-resolution observations made with the International LOFAR Telescope, which include baselines >1000 km. Suitable sources must contain significant correlated flux density (≳ 50 − 100 mJy) at frequencies around 110−190 MHz on scales of a few hundred milliarcseconds. At least for the 200−300-km international baselines, we find around 1 suitable calibrator source per square degree over a large part of the northern sky, in agreement with previous work. This should allow a randomly selected target to be successfully phase calibrated on the international baselines in over 50% of cases. Products of the survey include calibrator source lists and fringe-rate and delay maps of wide areas – typically a few degrees – around each source. The density of sources with significant correlated flux declines noticeably with baseline length over the range 200−600 km, with good calibrators on the longest baselines appearing only at the rate of 0.5 per sq. deg. Coherence times decrease from 1−3 min on 200-km baselines to about 1 min on 600-km baselines, suggesting that ionospheric phase variations contain components with scales of a few hundred kilometres. The longest median coherence time, at just over 3 min, is seen on the DE609 baseline, which at 227 km is close to being the shortest. We see median coherence times of between 80 and 110 s on the four longest baselines (580−600 km), and about 2 min for the other baselines. The success of phase transfer from calibrator to target is shown to be influenced by distance, in a manner that suggests a coherence patch at 150-MHz of the order of 1 deg. Although source structures cannot be measured in these observations, we deduce that phase transfer is affected if the calibrator source structure is not known. We give suggestions for calibration strategies and choice of calibrator sources, and describe the access to the online catalogue and data products.