The Low-Frequency Array (LOFAR) Long-Baseline Calibrator Survey (LBCS) was conducted between 2014 and 2019 in order to obtain a set of suitable calibrators for the LOFAR array. In this paper we present the complete survey, building on the preliminary analysis published in 2016 which covered approximately half the survey area. The final catalogue consists of 30006 observations of 24713 sources in the northern sky, selected for a combination of high low-frequency radio flux density and flat spectral index using existing surveys (WENSS, NVSS, VLSS, and MSSS). Approximately one calibrator per square degree, suitable for calibration of ≥ 200 km baselines is identified by the detection of compact flux density, for declinations north of 30 • and away from the Galactic plane, with a considerably lower density south of this point due to relative difficulty in selecting flat-spectrum candidate sources in this area of the sky. The catalogue contains indicators of degree of correlated flux on baselines between the Dutch core and each of the international stations, involving a maximum baseline length of nearly 2000 km, for all of the observations. Use of the VLBA calibrator list, together with statistical arguments by comparison with flux densities from lower-resolution catalogues, allow us to establish a rough flux density scale for the LBCS observations, so that LBCS statistics can be used to estimate compact flux densities on scales between 300 mas and 2 , for sources observed in the survey. The survey is used to estimate the phase coherence time of the ionosphere for the LOFAR international baselines, with median phase coherence times of about 2 minutes varying by a few tens of percent between the shortest and longest baselines. The LBCS can be used to assess the structures of point sources in lower-resolution surveys, with significant reductions in the degree of coherence in these sources on scales between 2 and 300 mas. The LBCS survey sources show a greater incidence of compact flux density in quasars than in radio galaxies, consistent with unified schemes of radio sources. Comparison with samples of sources from interplanetary scintillation (IPS) studies with the Murchison Widefield Array (MWA) shows consistent patterns of detection of compact structure in sources observed both interferometrically with LOFAR and using IPS.
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.
Relatively little information is available about the Universe at ultra-low radio frequencies (ULF; i.e., below 50 MHz), although the ULF spectral window contains a wealth of unique diagnostics for studying galactic and extragalactic phenomena. Subarcsecond resolution imaging at these frequencies is extremely difficult, due to the long baselines (>1000 km) required and large ionospheric perturbations. We have conducted a pilot project to investigate the ULF performance and potential of the International LOFAR Telescope (ILT), a trans-European interferometric array with baselines up to ~2000 km and observing frequencies down to 10 MHz. We have successfully produced images with subarcsecond resolution for six radio sources at frequencies down to 30 MHz. This resolution is more than an order of magnitude better than pre-ILT observations at similar frequencies. The six targets that we imaged (3C 196, 3C 225, 3C 273, 3C 295, 3C 298, and 3C 380) are bright radio sources with compact structures. By comparing our data of 3C 196 and 3C 273 with observations at higher frequencies, we investigate their spatially resolved radio spectral properties. Our success shows that at frequencies down to 30 MHz, subarcsecond imaging with the ILT is possible. Further analysis is needed to determine the feasibility of observations of fainter sources or sources with less compact emission.
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