Cassiopeia A, Cygnus A, Taurus A, and Virgo A at ultra-low radio
  frequencies
  The four persistent radio sources in the northern sky with the highest flux
density at metre wavelengths are Cassiopeia A, Cygnus A, Taurus A, and Virgo A;
collectively they are called the A-team. Their flux densities at ultra-low
frequencies (<100 MHz) can reach several thousands of janskys, and they often
contaminate observations of the low-frequency sky by interfering with image
processing. Furthermore, these sources are foreground objects for all-sky
observations hampering the study of faint signals, such as the cosmological 21
cm line from the epoch of reionisation.
  We aim to produce robust models for the surface brightness emission as a
function of frequency for the A-team sources at ultra-low frequencies. These
models are needed for the calibration and imaging of wide-area surveys of the
sky with low-frequency interferometers. This requires obtaining images at an
angular resolution better than 15 arcsec with a high dynamic range and good
image fidelity.
  We observed the A-team with the Low Frequency Array (LOFAR) at frequencies
between 30 MHz and 77 MHz using the Low Band Antenna (LBA) system. We reduced
the datasets and obtained an image for each A-team source.
  The paper presents the best models to date for the sources Cassiopeia A,
Cygnus A, Taurus A, and Virgo A between 30 MHz and 77 MHz. We were able to
obtain the aimed resolution and dynamic range in all cases. Owing to its
compactness and complexity, observations with the long baselines of the
International LOFAR Telescope will be required to improve the source model for
Cygnus A further.
