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Journal Articles Astronomy and Astrophysics - A&A Year : 2021

The LOFAR LBA Sky Survey: Deep Fields. I. The Boötes Field

W. L. Williams
  • Function : Author
F. de Gasperin
  • Function : Author
M. J. H. Hardcastle
  • Function : Author
R. van Weeren
  • Function : Author
T. W. Shimwell
  • Function : Author
P. N. Best
  • Function : Author
M. Bonato
  • Function : Author
M. Bondi
  • Function : Author
M. Brüggen
  • Function : Author
H. J. A. Röttgering
  • Function : Author
D. J. B. Smith
  • Function : Author


We present the first sub-mJy (≈0.7 mJy beam−1) survey to be completed below 100 MHz, which is over an order of magnitude deeper than previously achieved for widefield imaging of any field at these low frequencies. The high-resolution (15 × 15 arcsec) image of the Boötes field at 34-75 MHz is made from 56 hours of observation with the LOw Frequency ARray (LOFAR) Low Band Antenna (LBA) system. The observations and data reduction, including direction-dependent calibration, are described here. We present a radio source catalogue containing 1948 sources detected over an area of 23.6 deg2, with a peak flux density threshold of 5σ. Using existing datasets, we characterise the astrometric and flux density uncertainties, finding a positional uncertainty of ∼ 1.2 arcsec and a flux density scale uncertainty of about 5 per cent. Using the available deep 144-MHz data, we identified 144-MHz counterparts to all the 54-MHz sources, and produced a matched catalogue within the deep optical coverage area containing 829 sources. We calculate the Euclidean-normalised differential source counts and investigate the low-frequency radio source spectral indices between 54 and 144 MHz. Both show a general flattening in the radio spectral indices for lower flux density sources, from ∼ − 0.75 at 144-MHz flux densities between 100 and 1000 mJy to ∼ − 0.5 at 144-MHz flux densities between 5 and 10 mJy. Such flattening is attributable to a growing population of star forming galaxies and compact core-dominated AGN.

The image, full catalogue (Table 2), and matched catalogue (Sect. 5.1) are only available at the CDS via anonymous ftp to ( or via

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Dates and versions

insu-03720855 , version 1 (12-07-2022)



W. L. Williams, F. de Gasperin, M. J. H. Hardcastle, R. van Weeren, C. Tasse, et al.. The LOFAR LBA Sky Survey: Deep Fields. I. The Boötes Field. Astronomy and Astrophysics - A&A, 2021, 655, ⟨10.1051/0004-6361/202141745⟩. ⟨insu-03720855⟩
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