@article{292818b0505d41efbe780240a577f020,
title = "The LOFAR Two-metre Sky Survey (LoTSS). V. Second data release",
abstract = "In this data release from the ongoing LOw-Frequency ARray (LOFAR) Two-metre Sky Survey (LoTSS) we present 120-168 MHz images covering 27% of the northern sky. Our coverage is split into two regions centred at approximately 12h45m +44◦30′ and 1h00m +28◦00′ and spanning 4178 and 1457 square degrees respectively. The images were derived from 3,451 hrs (7.6 PB) of LOFARHigh 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 6′′ resolution, our full bandwidth Stokes I continuum maps with a central frequency of 144 MHz have: a median rms sensitivity of 83 μJy/beam; aflux density scale accuracy of approximately 10%; an astrometric accuracy of 0.2′′; and we estimate the point-source completeness to be 90% at a peak brightness of 0.8 mJy/beam. 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 > ±0.2 which is a consequence of our flux-density scale accuracy and small fractional bandwidth. Our circular polarisation (Stokes V) 20′′ resolution 120-168 MHz continuum images have a median rms sensitivity of 95 μJy/beam, 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 480 × 97.6 kHz wide planes and have a median rms sensitivity per plane of 10.8 mJy/beam at 4′ and 2.2 mJy/beam at 20′′; we estimate the Stokes I to Stokes Q/U leakage to be approximately0.2%. 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.",
keywords = "Catalogs, Radio continuum: general, Surveys, Techniques: image processing",
author = "{LOFAR collaboration} and Shimwell, {T W} and Best, {P. N.} and K. Duncan and Hale, {C. L.} and J. Sabater and L. Alegre and R. Kondapally",
note = "Funding Information: STFC through a ScotDIST Intensive Data Science Scholarship. A. Bonafede acknowledges support from ERC Stg {\textquoteleft}DRANOEL{\textquoteright} n. 714245 and MIUR FARE grant “SMS”. E. Bonnassieux acknowledges support from the ERC-Stg grant {\textquoteleft}DRANOEL{\textquoteright}, n.714245. M. Brienza acknowledges financial support from the ERC Stg “MAGCOW”, no. 714196 and from ERC Stg {\textquoteleft}DRA-NOEL{\textquoteright} no. 714245. M. Bilicki is supported by the Polish National Science Center through grants no. 2020/38/E/ST9/00395, 2018/30/E/ST9/00698 and 2018/31/G/ST9/03388, and by the Polish Ministry of Science and Higher Education through grant DIR/WK/2018/12. M. Bonato and I.P. acknowledge support from INAF under the SKA/CTA PRIN “FORECaST” and the PRIN MAIN STREAM “SAuROS” projects and from the Ministero degli Affari Esteri e della Cooperazione Internazionale – Direzione Generale per la Promozione del Sis-tema Paese Progetto di Grande Rilevanza ZA18GR02. V.C. acknowledges support from the Alexander von Humboldt Foundation. J.H.C. and B.M. acknowledge support from the UK Science and Technology Facilities Council (STFC) under grants ST/R00109X/1, ST/R000794/1, and ST/T000295/1. H.E. acknowledges supported by the DFG under grant number 427771150. K.E. is a Jansky Fellow of the National Radio Astronomy Observatory, USA. M. Haj acknowledges the MSHE for granting funds for Polish contribution to the International LOFAR Telescope (decision no. DIR/WK/2016/2017/05-1) and for maintenance of the LOFAR PL-612 Baldy (decision no. 59/E-383/SPUB/SP/2019.1), and the NAWA Bekker fellowship (grant No PPN/BEK/2019/1/00431). G.D.G. acknowledges support from the Alexander von Humboldt Foundation. D.N.H. acknowledges support from the ERC through the grant ERC-Stg {\textquoteleft}DRANOEL{\textquoteright} n. 714245. M.H. acknowledges support by the BMBF Verbundforschung under the grant 05A20STA. V.J. acknowledges support by the Croatian Science Foundation for a project IP-2018-01-2889 (LowFreqCRO). R.K. acknowledges support from the Science and Technology Facilities Council (STFC) through an STFC studentship via grant ST/R504737/1. M.K.B. and A.W. acknowledge support from the National Science Centre, Poland under grant no. 2017/26/E/ST9/00216. K.M. has been supported by the National Science Centre (UMO-2018/30/E/ST9/00082). F.M. is supported by the “Departments of Excellence 2018 – 2022” Grant awarded by the Italian Ministry of Education, University and Research (MIUR) (L. 232/2016). S.J.N. is supported by the Polish National Science Center through grant UMO-2018/31/N/ST9/03975. T.P. is supported by the BMBF Verbundforschung under grant number 50OR1906. K.R. acknowledges financial support from the ERC Starting Grant “MAGCOW”, no. 714196. C.J.R. acknowledges financial support from the ERC Starting Grant {\textquoteleft}DRANOEL{\textquoteright}, number 714245. M.V. acknowledges financial support from the Inter-University Institute for Data Intensive Astronomy (IDIA), a partnership of the University of Cape Town, the University of Pretoria, the University of the Western Cape and the South African Radio Astronomy Observatory, and from the South African Department of Science and Innovation{\textquoteright}s National Research Foundation under the ISARP RADIOSKY2020 Joint Research Scheme (DSI-NRF Grant Number 113121) and the CSUR HIPPO Project (DSI-NRF Grant Number 121291). X.Z. acknowledges support from China Scholarship Council The Dunlap Institute is funded through an endowment established by the David Dunlap family and the University of Toronto. J.L.W. acknowledges the support of the Natural Sciences and Engineering Research Council of Canada (NSERC) through grant RGPIN-2015-05948, and of the Canada Research Chairs program. Funding Information: own funding sources), and which are collectively operated by the ILT foundation under a joint scientific policy. The ILT resources have benefited from the following recent major funding sources: CNRS-INSU, Observatoire de Paris and Universit{\'e} d{\textquoteright}Orl{\'e}ans, France; BMBF, MIWF-NRW, MPG, Germany; Science Foundation Ireland (SFI), Department of Business, Enterprise and Innovation (DBEI), Ireland; NWO, The Netherlands; The Science and Technology Facilities Council, UK; Ministry of Science and Higher Education, Poland; The Istituto Nazionale di Astrofisica (INAF), Italy. This research made use of the Dutch national e-infrastructure with support of the SURF Cooperative (e-infra 180169) and NWO (grant 2019.056). The J{\"u}lich LOFAR Long Term Archive and the German LOFAR network are both coordinated and operated by the J{\"u}lich Supercomputing Centre (JSC), and computing resources on the supercomputer JUWELS at JSC were provided by the Gauss Centre for Supercomputing e.V. (grant CHTB00) through the John von Neumann Institute for Computing (NIC). This research made use of the University of Hertfordshire high-performance computing facility and the LOFAR-UK computing facility located at the University of Hertfordshire and supported by STFC [ST/P000096/1], and of the Italian LOFAR IT computing infrastructure supported and operated by INAF, and by the Physics Department of Turin university (under an agreement with Consorzio Interuniversitario per la Fisica Spaziale) at the C3S Supercomputing Centre, Italy. The data are published via the SURF Data Repository service which is supported by the EU funded DICE project (H2020-INFRAEOSC-2018-2020 under Grant Agreement no. 101017207). M.J.H. acknowledges support from STFC [ST/R000905/1, ST/V000624/1]. P.N.B. and J.S. are grateful for support from the UK STFC via grants ST/R000972/1 and ST/V000594/1. W.L.W. acknowledges support from the CAS-NWO programme for radio astronomy with project number 629.001.024, which is financed by the Netherlands Organisation for Scientific Research (NWO). A.B. acknowledges support from the VIDI research programme with project number 639.042.729, which is financed by the Netherlands Organisation for Scientific Research (NWO). A.D. acknowledges support by the BMBF Verbundforschung under the grant 05A20STA. R.J.vW. and R.T. acknowledge support from the ERC Starting Grant ClusterWeb 804208. M. Br{\"u}ggen and F.dG. acknowledge support from the Deutsche Forschungsge-meinschaft under Germany{\textquoteright}s Excellence Strategy – EXC 2121 “Quantum Universe” – 390833306. K.J.D. acknowledges funding from the European Union{\textquoteright}s Horizon 2020 research and innovation programme under the Marie Sk{\l}odowska-Curie grant agreement No. 892117 (HIZRAD). C.L.H. acknowledges support from the Leverhulme Trust through an Early Career Research Fellowship. M.H. acknowledges funding from the European Research Council (ERC) under the European Union{\textquoteright}s Horizon 2020 research and innovation programme (grant agreement No 772663). L.K.M. is grateful for support from the UKRI Future Leaders Fellowship (grant MR/T042842/1). D.J.S. and R.J.D. acknowledge support by the BMBF Verbundforschung under grant 05A20PB1 and 05A20PC3, respectively. The research of OS is supported by the South African Research Chairs Initiative of the Department of Science and Technology and National Research Foundation. G.J.W. gratefully acknowledges support of an Emeritus Fellowship from the Leverhulme Trust. L.A. acknowledges support from the Publisher Copyright: {\textcopyright} 2022 EDP Sciences. All rights reserved.",
year = "2022",
month = mar,
day = "1",
doi = "10.1051/0004-6361/202142484",
language = "English",
volume = "659",
pages = "1--27",
journal = "Astronomy and Astrophysics",
issn = "0004-6361",
publisher = "EDP Sciences",
}