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.

Accepted by Astronomy & Astrophysics. Error in figure 6 corrected

References in corpus (5)

• The VLA Low-frequency Sky Survey
• The Very Large Array Low-frequency Sky Survey Redux (VLSSr)
• Subarcsecond international LOFAR radio images of the M82 nucleus at 118 MHz and 154 MHz
• The LOFAR long baseline snapshot calibrator survey
• A deep, high resolution survey of the low frequency radio sky

Cited by in corpus (11)

• The LOFAR Two-metre Sky Survey - I. Survey Description and Preliminary Data Release
• Precise radio astrometry and new developments for the next generation of instruments
• The Faraday Rotation Measure Grid of the LOFAR Two-metre Sky Survey: Data Release 2
• Black hole jets on the scale of the Cosmic Web
• Interplanetary Scintillation studies with the Murchison Wide-field Array II: Properties of sub-arcsecond compact sources at low radio frequencies
• Interplanetary Scintillation with the Murchison Widefield Array I: A sub-arcsecond Survey over 900 square degrees at 79 and 158 MHz
• Measuring cavity powers of active galactic nuclei in clusters using a hybrid X-ray-radio method -- A new window on feedback opened by subarcsecond LOFAR-VLBI observations
• The LOFAR Two Metre Sky Survey: Deep Fields. II. The ELAIS-N1 LOFAR deep field
• LOFAR observations of 4C+19.44. On the discovery of low frequency spectral curvature in relativistic jet knots
• First measurement of interplanetary scintillation with the ASKAP radio telescope: implications for space weather
• Monster radio jet (>66 kpc) observed in quasar at z$\sim$5