Sub-arcsecond imaging with the International LOFAR Telescope I. Foundational calibration strategy and pipeline

Morabito LK, Jackson NJ, Mooney S, Sweijen F, Badole S, Kukreti P, Venkattu D, Groeneveld C, Kappes A, Bonnassieux E, Drabent A, Iacobelli M, Croston JH, Best PN, Bondi M, Callingham JR, Conway JE, Deller AT, Hardcastle MJ, Mckean JP, Miley GK, Moldon J, Rottgering HJA, Tasse C, Shimwell TW, Van Weeren RJ, Anderson JM, Asgekar A, Avruch IM, Van Bemmel IM, Bentum MJ, Bonafede A, Brouw WN, Butcher HR, Ciardi B, Corstanje A, Coolen A, Damstra S, De Gasperin F, Duscha S, Eisloeffel J, Engels D, Falcke H, Garrett MA, Griessmeier J, Gunst AW, Van Haarlem MP, Hoeft M, Van Der Horst AJ, Juette E, Kadler M, Koopmans LVE, Krankowski A, Mann G, Nelles A, Oonk JBR, Orru E, Paas H, Pandey VN, Pizzo RF, Pandey-Pommier M, Reich W, Rothkaehl H, Ruiter M, Schwarz DJ, Shulevski A, Soida M, Tagger M, Vocks C, Wijers RAMJ, Wijnholds SJ, Wucknitz O, Zarka P, Zucca P (2022)

Publication Type: Journal article

Publication year: 2022

Journal

Book Volume: 658

DOI: 10.1051/0004-6361/202140649

Abstract

The International LOFAR Telescope is an interferometer with stations spread across Europe. With baselines of up to similar to 2000 km, LOFAR has the unique capability of achieving sub-arcsecond resolution at frequencies below 200 MHz. However, it is technically and logistically challenging to process LOFAR data at this resolution. To date only a handful of publications have exploited this capability. Here we present a calibration strategy that builds on previous high-resolution work with LOFAR. It is implemented in a pipeline using mostly dedicated LOFAR software tools and the same processing framework as the LOFAR Two-metre Sky Survey (LoTSS). We give an overview of the calibration strategy and discuss the special challenges inherent to enacting high-resolution imaging with LOFAR, and describe the pipeline, which is publicly available, in detail. We demonstrate the calibration strategy by using the pipeline on P205+55, a typical LoTSS pointing with an 8 h observation and 13 international stations. We perform in-field delay calibration, solution referencing to other calibrators in the field, self-calibration of these calibrators, and imaging of example directions of interest in the field. We find that for this specific field and these ionospheric conditions, dispersive delay solutions can be transferred between calibrators up to similar to 1.5 degrees away, while phase solution transferral works well over similar to 1 degrees. We also demonstrate a check of the astrometry and flux density scale with the in-field delay calibrator source. Imaging in 17 directions, we find the restoring beam is typically similar to 0.3 '' x0.2 '' although this varies slightly over the entire 5 deg(2) field of view. We find we can achieve similar to 80-300 mu Jy bm(-1) image rms noise, which is dependent on the distance from the phase centre; typical values are similar to 90 mu Jy bm(-1) for the 8 h observation with 48 MHz of bandwidth. Seventy percent of processed sources are detected, and from this we estimate that we should be able to image roughly 900 sources per LoTSS pointing. This equates to similar to 3 million sources in the northern sky, which LoTSS will entirely cover in the next several years. Future optimisation of the calibration strategy for efficient post-processing of LoTSS at high resolution makes this estimate a lower limit.

Authors with CRIS profile

Involved external institutions

Netherlands Institute for Radio Astronomy Netherlands (NL) National Institute for Astrophysics / Istituto Nazionale Astrofisica (INAF) Italy (IT) Vrije Universiteit Brussel (VUB) Belgium (BE) Joint Institute for VLBI in Europe (JIVE) Netherlands (NL) University of Groningen / Rijksuniversiteit Groningen Netherlands (NL) Max-Planck-Gesellschaft zur Förderung der Wissenschaften e.V. (MPG) / Max Planck Society for the Advancement of Science Germany (DE) Mount Stromlo Observatory Australia (AU) Radboud University Nijmegen Netherlands (NL) Thüringer Landessternwarte Tautenburg (TLS) - Karl-Schwarzschild-Observatorium Germany (DE) University of Warmia and Mazury in Olsztyn Poland (PL) Ruhr-Universität Bochum (RUB) Germany (DE) Space Research Centre / Centrum Badań Kosmicznych Poland (PL) University of Orléans / Université d'Orléans France (FR) Université Claude Bernard Lyon 1 (UCB) France (FR) Leiden University Netherlands (NL) University of Hertfordshire United Kingdom (GB) University of Manchester United Kingdom (GB) University of Paris 7 - Denis Diderot / Université Paris VII Denis Diderot France (FR) Julius-Maximilians-Universität Würzburg Germany (DE) University of Amsterdam Netherlands (NL) Universität Hamburg (UHH) Germany (DE) Technische Universität Berlin Germany (DE) Spanish National Research Council / Consejo Superior de Investigaciones Científicas (CSIC) Spain (ES) Max-Planck-Institut für Radioastronomie / Max Planck Institute for Radio Astronomy Germany (DE) Jagiellonian University / Uniwersytet Jagielloński (UJ) Poland (PL) Leibniz Institute for Astrophysics Potsdam / Leibniz-Institut für Astrophysik Potsdam Germany (DE) Universität Bielefeld Germany (DE) Stockholm University / Stockholms universitet Sweden (SE) University College Dublin (UCD) Ireland (IE) University of Edinburgh United Kingdom (GB) Chalmers University of Technology / Chalmers tekniska högskola Sweden (SE) Swinburne University of Technology Australia (AU) George Washington University (GWU) United States (USA) (US) Open University (OU) United Kingdom (GB) SURF Netherlands (NL) University of Bologna / Università di Bologna Italy (IT) Durham University United Kingdom (GB)

How to cite

APA:

Morabito, L.K., Jackson, N.J., Mooney, S., Sweijen, F., Badole, S., Kukreti, P.,... Zucca, P. (2022). Sub-arcsecond imaging with the International LOFAR Telescope I. Foundational calibration strategy and pipeline. Astronomy & Astrophysics, 658. https://dx.doi.org/10.1051/0004-6361/202140649

MLA:

Morabito, L. K., et al. "Sub-arcsecond imaging with the International LOFAR Telescope I. Foundational calibration strategy and pipeline." Astronomy & Astrophysics 658 (2022).

BibTeX: Download