Context. The Low Frequency Array (LOFAR) is the only existing or planned radio interferometer able to observe at ultra-low frequencies (< 100 MHz) with high-resolution (< 15 ′′ ) and high-sensitivity (< 1 mJy beam −1 ). Capitalising on these capabilities, the LOFAR Surveys Key Science Project is realising two wide area surveys using the two LOFAR antenna types: the LOFAR Two Metre Sky Survey (LoTSS; at 120 − 168 MHz using the High Band Antenna) and the LOFAR LBA Sky Survey (LoLSS; at 42 − 66 MHz using the Low Band Antenna). Aims. With most of the data taken, LoLSS will eventually cover all the northern sky at Dec > 24 • reaching a resolution of 15 ′′ at 1 − 2 mJy beam −1 (1σ rms noise) depending on declination, field properties, and observing conditions. Here we present the first data release, including the calibration strategy and the properties of the released images and catalogues. Methods. A fully automated pipeline was used to reduce the 95 pointings that are part of this release. The data reduction procedures developed for this project are general and currently being used for almost any LOFAR LBA interferometric observation. Compared to the preliminary release, direction-dependent errors are now derived and included in the calibration. This allows to reach a typical sensitivity of 1.55 mJy beam −1 , four times higher than the preliminary release, at the target resolution of 15 ′′ . Results. The first release of the LOFAR LBA Sky Survey covers 650 deg 2 in the HETDEX region. We release to the scientific community the mosaic images (I and V Stokes) of the region, as well as a catalogue of 42,463 sources and relative Gaussian components. Catalogue of six in-band frequency channels at 44, 48, 52, 56, 60, and 64 MHz are also released. Conclusions. The first data release of LoLSS proves that modern radio interferometer can do large scale surveys at ultra-low frequency with high sensitivity and resolution. The data can be used to derive unique information on the low-frequency spectral properties of thousands of sources.
Context. The Low Frequency Array (LOFAR) is the only existing radio interferometer able to observe at ultra-low frequencies (< 100 MHz) with high resolution (< 15 ) and high sensitivity (< 1 mJy beam −1 ). 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 − 66 MHz named the LOFAR LBA Sky Survey (LoLSS). Aims. LoLSS is covering the whole northern sky above declination 24 • with a resolution of 15 and a sensitivity of 1 − 2 mJy beam −1 (1σ) 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. Methods. 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 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 1.55 mJy beam −1 , four times better than for the preliminary release, at the target resolution of 15 . Results. The first data release of the LOFAR LBA Sky Survey covers 650 deg 2 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 42 463 detected sources and related Gaussian components used to describe sources' morphologies. Separate catalogues for the 6 in-band frequencies of 44, 48, 52, 56, 60, and 64 MHz are also released. Conclusions. 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 − 66 MHz 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.
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