The LOFAR long baseline snapshot calibrator survey
Journal
Date Issued
2015
Author(s)
Moldón, J.
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Deller, A. T.
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Wucknitz, O.
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Jackson, N.
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Drabent, A.
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Carozzi, T.
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Conway, J.
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Kapińska, A. D.
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McKean, J. P.
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Morabito, L.
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Varenius, E.
•
Zarka, P.
•
Anderson, J.
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Asgekar, A.
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Avruch, I. M.
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Bell, M. E.
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Bentum, M. J.
•
•
Best, P.
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Bîrzan, L.
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Bregman, J.
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Breitling, F.
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Broderick, J. W.
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Brüggen, M.
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Butcher, H. R.
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Carbone, D.
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Ciardi, B.
•
•
de Geus, E.
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Duscha, S.
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Eislöffel, J.
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Engels, D.
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Falcke, H.
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Fallows, R. A.
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Fender, R.
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Ferrari, C.
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Frieswijk, W.
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Garrett, M. A.
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Grießmeier, J.
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Gunst, A. W.
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Hamaker, J. P.
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Hassall, T. E.
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Heald, G.
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Hoeft, M.
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Juette, E.
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Karastergiou, A.
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Kondratiev, V. I.
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Kramer, M.
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Kuniyoshi, M.
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Kuper, G.
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Maat, P.
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Mann, G.
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Markoff, S.
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McFadden, R.
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McKay-Bukowski, D.
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Morganti, R.
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Munk, H.
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Norden, M. J.
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Offringa, A. R.
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Orru, E.
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Paas, H.
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Pandey-Pommier, M.
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Pizzo, R.
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Polatidis, A. G.
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Reich, W.
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Röttgering, H.
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Rowlinson, A.
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Scaife, A. M. M.
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Schwarz, D.
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Sluman, J.
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Smirnov, O.
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Stappers, B. W.
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Steinmetz, M.
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Tagger, M.
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Tang, Y.
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Tasse, C.
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Thoudam, S.
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Toribio, M. C.
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Vermeulen, R.
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Vocks, C.
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van Weeren, R. J.
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White, S.
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Wise, M. W.
•
Yatawatta, S.
•
Zensus, A.
Description
LOFAR, the Low Frequency Array designed and constructed by ASTRON, has facilities in several countries that are owned by various parties (each with their own funding sources), and that are collectively operated by the international LOFAR Telescope (ILT) foundation under a joint scientific policy. A.T.D. is supported by a Veni Fellowship from NWO. A.D.K. acknowledges support from the Australian Research Council Centre of Excellence for All-sky Astrophysics (CAASTRO), through project number CE110001020. LKM acknowledges financial support from NWO Top LOFAR project, project n. 614.001.006. CF acknowledges financial support by the Agence Nationale de la Recherche through grant ANR-09-JCJC-0001-01.
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.
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.
Full Table 6 is only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/574/A73
Volume
574
Start page
A73
Issn Identifier
0004-6361
Ads BibCode
2015A&A...574A..73M
Rights
open.access
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