LBCS: the LOFAR Long-Baseline Calibrator Survey
Abstract
(abridged). We outline LBCS (the LOFAR Long-Baseline Calibrator Survey), 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 (50-100mJy) at frequencies around 110--190~MHz on scales of a few hundred mas. 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 square degree. Coherence times decrease from 1--3 minutes on 200-km baselines to about 1 minute 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 minutes, is seen on the DE609 baseline, which at 227km is close to being the shortest. We see median coherence times of between 80 and 110 seconds on the four longest baselines (580--600~km), and about 2 minutes 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 degree.
Keywords
Cite
@article{arxiv.1608.02133,
title = {LBCS: the LOFAR Long-Baseline Calibrator Survey},
author = {N. Jackson and A. Tagore and A. Deller and J. Moldón and E. Varenius and L. Morabito and O. Wucknitz and T. Carozzi and J. Conway and A. Drabent and A. Kapinska and E. Orrù and M. Brentjens and R. Blaauw and G. Kuper and J. Sluman and J. Schaap and N. Vermaas and M. Iacobelli and L. Cerrigone and A. Shulevski and S. ter Veen and R. Fallows and R. Pizzo and M. Sipior and J. Anderson and M. Avruch and M. Bell and I. van Bemmel and M. Bentum and P. Best and A. Bonafede and F. Breitling and J. Broderick and W. Brouw and M. Brüggen and B. Ciardi and A. Corstanje and F. de Gasperin and E. de Geus and J. Eislöffel and D. Engels and H. Falcke and M. Garrett and J. Griessmeier and A. Gunst and M. van Haarlem and G. Heald and M. Hoeft and J. Hörandel and A. Horneffer and H. Intema and E. Juette and M. Kuniyoshi and J. van Leeuwen and G. Loose and P. Maat and R. McFadden and D. McKay-Bukowski and J. McKean and D. Mulcahy and H. Munk and M. Pandey-Pommier and A. Polatidis and W. Reich and H. Röttgering and A. Rowlinson and A. Scaife and D. Schwarz and M. Steinmetz and J. Swinbank and S. Thoudam and M. Toribio and R. Vermeulen and C. Vocks and R. van Weeren and M. Wise and S. Yatawatta and P. Zarka},
journal= {arXiv preprint arXiv:1608.02133},
year = {2016}
}
Comments
Accepted by Astronomy & Astrophysics. Error in figure 6 corrected