1. |
- Shimwell, T. W., et al.
(author)
-
The LOFAR Two-metre Sky Survey: II. First data release
- 2019
-
In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 622
-
Research review (peer-reviewed)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.
|
|
2. |
- Shimwell, T. W., et al.
(author)
-
The LOFAR Two-metre Sky Survey: V. Second data release
- 2022
-
In: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 659
-
Journal article (peer-reviewed)abstract
- In this data release from the ongoing LOw-Frequency ARray (LOFAR) Two-metre Sky Survey we present 120a 168 MHz images covering 27% of the northern sky. Our coverage is split into two regions centred at approximately 12h45m +44 30a and 1h00m +28 00a and spanning 4178 and 1457 square degrees respectively. The images were derived from 3451 h (7.6 PB) of LOFAR High Band Antenna data which were corrected for the direction-independent instrumental properties as well as direction-dependent ionospheric distortions during extensive, but fully automated, data processing. A catalogue of 4 396 228 radio sources is derived from our total intensity (Stokes I) maps, where the majority of these have never been detected at radio wavelengths before. At 6a resolution, our full bandwidth Stokes I continuum maps with a central frequency of 144 MHz have: a median rms sensitivity of 83 μJy beama 1; a flux density scale accuracy of approximately 10%; an astrometric accuracy of 0.2a; and we estimate the point-source completeness to be 90% at a peak brightness of 0.8 mJy beama 1. By creating three 16 MHz bandwidth images across the band we are able to measure the in-band spectral index of many sources, albeit with an error on the derived spectral index of > a ±a 0.2 which is a consequence of our flux-density scale accuracy and small fractional bandwidth. Our circular polarisation (Stokes V) 20a resolution 120a168 MHz continuum images have a median rms sensitivity of 95 μJy beama 1, and we estimate a Stokes I to Stokes V leakage of 0.056%. Our linear polarisation (Stokes Q and Stokes U) image cubes consist of 480a A a 97.6 kHz wide planes and have a median rms sensitivity per plane of 10.8 mJy beama 1 at 4a and 2.2 mJy beama 1 at 20a; we estimate the Stokes I to Stokes Q/U leakage to be approximately 0.2%. Here we characterise and publicly release our Stokes I, Q, U and V images in addition to the calibrated uv-data to facilitate the thorough scientific exploitation of this unique dataset.
|
|
3. |
- O'Sullivan, S. P., et al.
(author)
-
The Faraday Rotation Measure Grid of the LOFAR Two-metre Sky Survey: Data Release 2
- 2023
-
In: Monthly Notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 519:4, s. 5723-5742
-
Journal article (peer-reviewed)abstract
- A Faraday rotation measure (RM) catalogue, or RM Grid, is a valuable resource for the study of cosmic magnetism. Using the second data release (DR2) from the LOFAR Two-metre Sky Survey (LoTSS), we have produced a catalogue of 2461 extragalactic high-precision RM values across 5720 deg(2) of sky (corresponding to a polarized source areal number density of similar to 0.43 deg(-2)). The linear polarization and RM properties were derived using RM synthesis from the Stokes Q and U channel images at an angular resolution of 20 arcsec across a frequency range of 120 to 168 MHz with a channel bandwidth of 97.6 kHz. The fraction of total intensity sources (>1 mJy beam(-1)) found to be polarized was similar to 0.2 percent. The median detection threshold was 0.6 mJy beam(-1) (8 sigma(QU)), with a median RM uncertainty of 0.06 rad m(-2) (although a systematic uncertainty of up to 0.3 radm(-2) is possible, after the ionosphere RM correction). The median degree of polarization of the detected sources is 1.8 percent, with a range of 0.05 percent to 31 percent. Comparisons with cm-wavelength RMs indicate minimal amounts of Faraday complexity in the LoTSS detections, making them ideal sources for RM Grid studies. Host galaxy identifications were obtained for 88 percent of the sources, along with redshifts for 79 percent (both photometric and spectroscopic), with the median redshift being 0.6. The focus of the current catalogue was on reliability rather than completeness, and we expect future versions of the LoTSS RM Grid to have a higher areal number density. In addition, 25 pulsars were identified, mainly through their high degrees of linear polarization.
|
|