LBCS：LOFAR长期基线校准调查
LBCS: the LOFAR Long-Baseline Calibrator Survey
摘要：
（节略）。我们概述了LBCS（LOFAR长基线校准测量），其目的是确定适合校准国际LOFAR望远镜（包括基线>1000 km）最高分辨率观测的源。合适的源必须在110- 190 MHz附近的频率上在几百mas的尺度上包含显著的相关通量密度（50- 100 mJ y）。至少对于200- 300公里的国际基线，我们发现在北方天空的大部分区域每平方度大约有1个合适的校准源，这与以前的工作一致。这将使随机选择的目标在50%以上的情况下成功地在国际基线上进行相位校准。调查的产品包括校准源列表和边缘率和延迟地图的广泛地区-通常是几度-围绕每个源。在200- 600 km范围内，具有显著相关通量的源密度随基线长度的增加而显著下降，在最长基线上，好的校准源仅以每平方度0.5个的速率出现。相干时间从200公里基线上的1- 3分钟减少到600公里基线上的约1分钟，表明电离层相位变化包含几百公里尺度的成分。在DE 609基线上看到的最长的中值相干时间仅为3分钟多一点，在227公里处接近最短。我们看到在四个最长的基线（580- 600 km）上的中位相干时间在80和110秒之间，而在其他基线上约为2分钟。从校准器到目标的相位转移的成功被示出为受距离的影响，在一种方式，建议在150 MHz的1度的顺序的相干补丁。
英文摘要：
(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.
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