2016A&A...595A..86J

We outline the LOFAR Long-Baseline Calibrator Survey (LBCS), whose aim is to identify sources suitable for calibrating the highest-resolution observations made with the International LOFAR Telescope, which include baselines >1000km. Suitable sources must contain significant correlated flux density (≥50-100mJy) at frequencies around 110-190MHz on scales of a few hundred milliarcseconds. At least for the 200-300km 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 200-600km, with good calibrators on the longest baselines appearing only at the rate of 0.5 per sq. deg. Coherence times decrease from 1-3min on 200km baselines to about 1min on 600km baselines, suggesting that ionospheric phase variations contain components with scales of a few hundred kilometres. The longest median coherence time, at just over 3min, is seen on the DE609 baseline, which at 227km is close to being the shortest. We see median coherence times of between 80 and 110s on the four longest baselines (580-600km), and about 2min 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°. Although source structures cannot be measured in these observations, we deduce that phase transfer is affected if the calibrator source structure is not known. We give suggestions for calibration strategies and choice of calibrator sources, and describe the access to the online catalogue and data products.