The LOFAR long baseline snapshot calibrator survey

J. Moldón 1 A. T. Deller O. Wucknitz 2 N. Jackson 3 A. Drabent 4 T. Carozzi 5 J. Conway 6 A. D. Kapińska 7 P. Mckean 1 L. Morabito 8 E. Varenius 6 P. Zarka 9 J. Anderson 10 A. Asgekar 1 I. M. Avruch 11 M. E. Bell 12 M. J. Bentum 1 G. Bernardi P. Best 13 L. Bîrzan 14 J. Bregman 1 F. Breitling 15 J. W. Broderick 16 M. Brüggen 17 H. R. Butcher 18 D. Carbone 19 B. Ciardi 20 F. de Gasperin 21 E. de Geus 22 S. Duscha 1 J. Eislöffel 4 D. Engels 21 H. Falcke 23 R. A. Fallows 24 R. Fender 16 C. Ferrari 25 W. Frieswijk M. A. Garrett 14 Jean-Mathias Grießmeier 26, 27 A. W. Gunst 1 J. P. Hamaker 1 T. E. Hassall 3 G. Heald M. Hoeft 4 E. Juette 28 A. Karastergiou 29 V. I. Kondratiev M. Kramer 30, 31 M. Kuniyoshi 30 G. Kuper P. Maat G. Mann 32 S. Markoff 33 R. Mcfadden 1 D. Mckay-Bukowski 34 R. Morganti 35, 1 H. Munk 1 M. J. Norden A. R. Offringa 35, 36 E. Orru H. Paas 37 M. Pandey-Pommier 38 R. Pizzo A. G. Polatidis W. Reich 30 H. Röttgering 14 A. Rowlinson A. M. M. Scaife 39 D. Schwarz 40 J. Sluman O. Smirnov 41 B. W. Stappers 3 M. Steinmetz 42 Michel Tagger 27 Y. Tang 43 C. Tasse 44, 45 S. Thoudam 23 M. C. Toribio 46 R. Vermeulen C. Vocks 32 R. J. van Weeren S. White M. W. Wise 33 S. Yatawatta A. Zensus 30
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Abstract : Aims. An efficient means of locating calibrator sources for International 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 pre-selected 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. Over 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 density of calibrators on the sky 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.
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J. Moldón, A. T. Deller, O. Wucknitz, N. Jackson, A. Drabent, et al.. The LOFAR long baseline snapshot calibrator survey. Astronomy and Astrophysics - A&A, EDP Sciences, 2015, 574, pp.A73. ⟨10.1051/0004-6361/201425042⟩. ⟨insu-01323110⟩

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