The Low Frequency Array (LOFAR) is the only existing radio interferometer able to observe at ultra-low frequencies (<100MHz) with high resolution (<15-arcsec) and high sensitivity (<1mJy/beam). To exploit these capabilities, the LOFAR Surveys Key Science Project is using the LOFAR Low Band Antenna (LBA) to carry out a sensitive wide-area survey at 41-66MHz named the LOFAR LBA Sky Survey (LoLSS). LoLSS is covering the whole northern sky above declination 24 deg with a resolution of 15-arcsec and a sensitivity of 1-2mJy/beam (1 {sigma}) depending on declination, field properties, and observing conditions. Here we present the first data release, including a discussion of the calibration strategy and the properties of the released images and catalogues. A fully automated pipeline was used to reduce the 95 fields included in this data release. The data reduction procedures developed for this project have a general application and are currently being used to process almost all LOFAR LBA interferometric observations. Compared to the preliminary release, direction-dependent errors have been derived and corrected for during the calibration process. This results in a typical sensitivity of 1.55mJy/beam, which is four times better than for the preliminary release, at the target resolution of 15 arcsec. The first data release of the LOFAR LBA Sky Survey covers 650 sqdeg in the HETDEX spring field. The resultant data products released to the community include mosaic images (I and V Stokes) of the region, and a catalogue of 42463 detected sources and related Gaussian components used to describe the sources' morphologies. Separate catalogues for the six in-band frequencies of 44, 48, 52, 56, 60, and 64MHz are also released. The first data release of LoLSS shows that, despite the influences of the ionosphere and radio frequency interference, LOFAR can conduct large-scale surveys in the frequency window 42-66MHz with unprecedentedly high sensitivity and resolution. The data can be used to derive unique information on the low-frequency spectral properties of many thousands of sources with a wide range of applications in extragalactic and galactic astronomy.

Cone search capability for table J/A+A/673/A165/lolss1s (LoLSS DR1 v1.1 catalogue (LoLSS]_DR1_v1.1.srl))

Cone search capability for table J/A+A/673/A165/lolss1g (LoLSS DR1 v1.1 catalogue with Gaussians (LoLSS]_DR1_v1.1.gaus))