The LOFAR Two-metre Sky Survey

Abstract

The LOFAR Two-metre Sky Survey (LoTSS) is an ongoing sensitive, high-resolution 120–168 MHz survey of the entire northern sky for which observations are now 20% complete. We present our first full-quality public data release. For this data release 424 square degrees, or 2% of the eventual coverage, in the region of the HETDEX Spring Field (right ascension 10h45m00s to 15h30m00s and declination 45°00′00″ to 57°00′00″) were mapped using a fully automated direction-dependent calibration and imaging pipeline that we developed. A total of 325 694 sources are detected with a signal of at least five times the noise, and the source density is a factor of ∼10 higher than the most sensitive existing very wide-area radio-continuum surveys. The median sensitivity is S144 MHz = 71 μJy beam−1 and the point-source completeness is 90% at an integrated flux density of 0.45 mJy. The resolution of the images is 6″ and the positional accuracy is within 0.2″. This data release consists of a catalogue containing location, flux, and shape estimates together with 58 mosaic images that cover the catalogued area. In this paper we provide an overview of the data release with a focus on the processing of the LOFAR data and the characteristics of the resulting images. In two accompanying papers we provide the radio source associations and deblending and, where possible, the optical identifications of the radio sources together with the photometric redshifts and properties of the host galaxies. These data release papers are published together with a further ∼20 articles that highlight the scientific potential of LoTSS.

Fundamental differences in the radio properties of red and blue quasars: insight from the LOFAR Two-metre Sky Survey (LoTSS)

“…The LOFAR Two-metre Sky Survey is an on-going programme to image the entire northern sky in the 120-168 MHz radio band at a resolution of 6" (Shimwell et al 2017). The first full resolution data release (DR1; Shimwell et al 2019) covers 424 deg 2 in the HETDEX spring field (RA: 10h45m -15h30m, Dec: 45 • -57 • ), which lies completely within the SDSS footprint. Calibrated and synthesised radio images, in the form of science mosaics and root-meansquare (RMS) noise maps, and a combined source catalogue are available from the LOFAR surveys public releases page.…”

LOFAR 144-MHz follow-up observations of GW170817

“…(iv) We also demonstrated that it is possible to obtain images with LOFAR significantly south of the celestial equator, albeit a factor of about 1.5 dex less sensitive and at an angular resolution 2.5-5.3 times coarser than what is achievable at or near zenith, in this particular case. If LoTSS were to be extended below the celestial equator, with an angular resolution at or near the usual target value of 6 arcsec (Shimwell et al , 2019, this would allow high-resolution, lowfrequency sky models to be developed at declinations that will be readily accessible with the first phase of the lowfrequency component of the Square Kilometre Array (i.e. SKA1-LOW).…”

“…In principle, however, Run 2 still had sufficiently fine temporal and frequency resolutions to permit proper calibration, as was the case for Run 1 (e.g. see Shimwell et al 2017Shimwell et al , 2019.…”

“…The reduction steps after preprocessing were directionindependent calibration, followed by direction-dependent calibration and imaging. A detailed description of the procedure, which was developed for the LOFAR Two-metre Sky Survey (LoTSS), can be found in Shimwell et al (2017Shimwell et al ( , 2019. The direction-independent pipeline removed the effects outlined in de Gasperin et al (2019) The direction-dependent step made use of kms (Tasse 2014a,b;Smirnov & Tasse 2015) and ddfacet (Tasse et al 2018) for calibration and imaging, respectively.…”

“…While our LOFAR upper limits lie well above the two curves, we can consider the hypothetical scenario of LOFAR late-time flux density measurements had an event similar to GW170817 been much further north on the sky. LOFAR can achieve a median noise level of approximately 70 µJy beam −1 in routine 8-h observations, with 48 MHz bandwidth centred at 144 MHz, for declinations at or near zenith (Shimwell et al 2019). Considering the radio light curve fitting in Mooley et al (2018c) and also Alexander et al (2018) (in addition, see Dobie et al 2018), the peak flux density at 144 MHz would be predicted to be at approximately 7-9.5 times the median LOFAR sensitivity level (see left panel of Figure 2).…”