The LOFAR Two-metre Sky Survey: VII. Third Data Release

Abstract

We present the third data release of the LOFAR Two-metre Sky Survey (LoTSS-DR3). The survey images cover 88% of the northern sky and were created from 12,950 hrs of data (18.6 PB) accumulated over 10.5 years. The images were produced through direction-independent and direction-dependent calibration pipelines that correct for instrumental effects as well as spatially and temporally varying ionospheric distortions. In our 120-168 MHz continuum mosaic images with an angular resolution of 6 arcsec (9 arcsec below declination 10$^\circ$) we catalogue 13,667,877 sources, formed from 16,943,656 Gaussian components. The scatter in the astrometric precision approximately follows the expected noise-like behaviour but with an additional systematic component of at least 0.24 arcsec that is likely due to calibration imperfections. The random flux density scale error is 6%, while the systematic offset was previously shown to be within 2%. The median sensitivity of our mosaics is 92$μ$Jy beam$^{-1}$. Completeness simulations, accounting for realistic source models, time- and bandwidth-smearing effects, and astrometric errors, indicate that we detect more than 95% of compact sources with integrated flux densities exceeding 9 times the local root mean square (RMS) noise. However, the recovered source counts in a particular integrated flux density bin do not match the injected counts until flux densities exceed 45 times the local RMS noise. The Euclidean-normalised differential source counts derived from the survey constrain the radio source population over five orders of magnitude and are in good agreement with previous deep and wide-area surveys. All data products are publicly available, including catalogues, individual-field Stokes I, Q, U, and V images, mosaicked Stokes I images, and $uv$ data with associated direction-dependent calibration solutions.

Figures (15)

• Figure 1: Top: Re-projection of the LoTSS-DR3 mosaic images. Bottom: Corresponding RMS image. The yellow and blue outlines show the LoTSS-DR1 and LoTSS-DR2 areas, which cover 2% and 27% of the northern sky, respectively. The black outline in the bottom panel shows the LoTSS-DR3 coverage of 88% of the northern sky. The large grey circles show regions that are within 10$^\circ$ of the bright radio sources Cassiopeia A, Cygnus A, Taurus A, Hercules A, or Virgo A. The small grey dots show the locations of the 3,168 LoTSS pointings of which 2,551 are included in this data release.
• Figure 2: Breakdown of the computational time for a typical DDF-pipeline run on a standard 8 hr observation with 48 MHz bandwidth. The total runtime is 75 hrs using 60 CPU cores on a node with AMD EPYC Bergamo processors (128 cores per socket). The runtime includes the creation of all Stokes I, Q, U, and V science products, five Stokes I imaging cycles, and four calibration cycles.
• Figure 3: Distribution of the mean RA (top) and Dec (bottom) astrometric offsets between LoTSS-DR3 and FIRST for the different LoTSS-DR3 individual pointing and mosaic catalogues. For the individual pointing catalogues, we show the distribution of mean offsets both before and after the astrometric corrections described in Sec. \ref{['Sec:astrometric']} were applied.
• Figure 4: Flux density scale alignment of LoTSS-DR3. Top: Median ratio of the catalogued integrated flux density between pointing P028+41 and the neighbouring fields before any correction to the flux density scale has been applied. The colour background shows the plane fit to the integrated flux density ratios that we use to align this pointing with the neighbouring fields. The colour bar corresponds to both the points and the background. Bottom: How the distribution of the median integrated flux density scale ratio changes as different iterations of the alignment corrections are progressively applied. The standard deviations of the fitted Gaussian functions are displayed in the legend.
• Figure 5: Top: Individual pointing 'matched RMS' as a function of observation date (green) together with the raw RMS measured from the images. The 'matched RMS' is the raw RMS adjusted to account for observation elevation, duration and bandwidth differences; it is scaled to match an 8 hr 231 sub-band observation at optimal elevation ($\sigma_{Matched} = \sigma_{Measured} \times \cos(\pi/2 - \epsilon)^2 \times \left( \frac{(1-F)\times T \times N_{SB}}{8.0\times231} \right)^{0.5}$ ), where $\epsilon$ is the elevation, $F$ is the fraction flagged, $T$ is the observation duration in hours, and $N_{SB}$ is the number of sub-bands. Also plotted is the flagging fraction and the daily total number of sunspots (Clette_2015) which gives an indication of the solar activity throughout the observing range. Middle: RMS adjusted to account for observation duration and bandwidth demonstrating that the expected elevation dependence is observed. Bottom: Histogram of the raw and matched RMS values.