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

Jackson, Neal and Badole, Shruti and Morgan, John and Chhetri, Rajan and Prūsis, Kaspars and Nikolajevs, Atvars and Morabito, Leah and Brentjens, Michiel and Sweijen, Frits and Iacobelli, Marco and Orrù, Emanuela and Sluman, J. and Blaauw, R. and Mulder, H. and van Dijk, P. and Mooney, Sean and Deller, Adam and Moldon, Javier and Callingham, J.R. and Harwood, Jeremy and Hardcastle, Martin and Heald, George and Drabent, Alexander and McKean, J.P. and Asgekar, A. and Avruch, I.M. and Bentum, M.J. and Bonafede, A. and Brouw, W.N. and Brüggen, M. and Butcher, H.R. and Ciardi, B. and Coolen, A. and Corstanje, A. and Damstra, S. and Duscha, S. and Eislöffel, J. and Falcke, H. and Garrett, M. and de Gasperin, F. and Griessmeier, J.-M. and Gunst, A.W. and van Haarlem, M.P. and Hoeft, M. and van der Horst, A.J. and Jütte, E. and Koopmans, L.V.E. and Krankowski, A. and Maat, P. and Mann, G. and Miley, G.K. and Nelles, A. and Norden, M. and Paas, M. and Pandey, V.N. and Pandey-Pommier, M. and Pizzo, R.F. and Reich, W. and Rothkaehl, H. and Rowlinson, A. and Ruiter, M. and Shulevski, A. and Schwarz, D.J. and Smirnov, O. and Tagger, M. and Vocks, C. and van Weeren, R.J. and Wijers, R. and Wucknitz, O. and Zarka, P. and Zensus, J.A. and Zucca, P. (2022) 'Sub-arcsecond imaging with the International LOFAR Telescope II. Completion of the LOFAR Long-Baseline Calibrator Survey.', Astronomy & Astrophysics, 658 . A2.


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.

Item Type:Article
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Publisher statement:Reproduced with permission, © ESO.
Date accepted:29 April 2021
Date deposited:30 June 2021
Date of first online publication:27 May 2021
Date first made open access:30 June 2021

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