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 13 P. Best 14 L. Bîrzan 15 J. Bregman 1 F. Breitling 16 J. W. Broderick 17 M. Brüggen 18 H. R. Butcher 19 D. Carbone 20 B. Ciardi 21 F. de Gasperin 22 E. de Geus 23 S. Duscha 1 J. Eislöffel 4 D. Engels 22 H. Falcke 24 R. A. Fallows 25 R. Fender 17 C. Ferrari 26 W. Frieswijk M. A. Garrett 15 Jean-Mathias Grießmeier 27, 28 A. W. Gunst 1 J. P. Hamaker 1 T. E. Hassall 3 G. Heald M. Hoeft 4 E. Juette 29 A. Karastergiou 30 V. I. Kondratiev M. Kramer 31, 32 M. Kuniyoshi 31 G. Kuper P. Maat G. Mann 33 S. Markoff 34 R. Mcfadden 1 D. Mckay-Bukowski 35 R. Morganti 36, 37 H. Munk 1 M. J. Norden A. R. Offringa 36, 38 E. Orru H. Paas 39 M. Pandey-Pommier 40 R. Pizzo A. G. Polatidis W. Reich 31 H. Röttgering 15 A. Rowlinson A. M. M. Scaife 41 D. Schwarz 42 J. Sluman O. Smirnov 43 B. W. Stappers 3 M. Steinmetz 44 Michel Tagger 28 Y. Tang 45 C. Tasse 46, 47 S. Thoudam 24 M. C. Toribio 48 R. Vermeulen C. Vocks 33 R. J. Van Weeren S. White M. W. Wise 34 S. Yatawatta A. Zensus 31
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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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