The LOFAR LBA Sky Survey II. First data release

Abstract

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.

The Decameter sky at sub-arcminute resolution

“…The core of LOFAR comprises baselines up to 120 km in the Nether-lands. Previous studies with LOFAR consist of, among others, the LOFAR Two-meter Sky Survey (LoTSS,120-168 MHz Shimwell et al 2017 and the LOFAR LBA Sky Survey (LoLSS, 42-66 MHz, de Gasperin et al 2021, 2023. The sensitivity, resolution and frequency coverage of these observations, along with previous low-frequency work, are shown in Figure 1.…”