Sub-arcsecond imaging with the International LOFAR Telescope: II.
  Completion of the LOFAR Long-Baseline Calibrator Survey
  The Low-Frequency Array (LOFAR) Long-Baseline Calibrator Survey (LBCS) was
conducted between 2014 and 2019 in order to obtain a set of suitable
calibrators for the LOFAR array. In this paper we present the complete survey,
building on the preliminary analysis published in 2016 which covered
approximately half the survey area. The final catalogue consists of 30006
observations of 24713 sources in the northern sky, selected for a combination
of high low-frequency radio flux density and flat spectral index using existing
surveys (WENSS, NVSS, VLSS, and MSSS). Approximately one calibrator per square
degree, suitable for calibration of $\geq$ 200 km baselines is identified by
the detection of compact flux density, for declinations north of 30 degrees and
away from the Galactic plane, with a considerably lower density south of this
point due to relative difficulty in selecting flat-spectrum candidate sources
in this area of the sky. Use of the VLBA calibrator list, together with
statistical arguments by comparison with flux densities from lower-resolution
catalogues, allow us to establish a rough flux density scale for the LBCS
observations, so that LBCS statistics can be used to estimate compact flux
densities on scales between 300 mas and 2 arcsec, for sources observed in the
survey. The LBCS can be used to assess the structures of point sources in
lower-resolution surveys, with significant reductions in the degree of
coherence in these sources on scales between 2 arcsec and 300 mas. The LBCS
survey sources show a greater incidence of compact flux density in quasars than
in radio galaxies, consistent with unified schemes of radio sources. Comparison
with samples of sources from interplanetary scintillation (IPS) studies with
the Murchison Widefield Array (MWA) shows consistent patterns of detection of
compact structure in sources observed both interferometrically with LOFAR and
using IPS.
