Sub-arcsecond imaging with the International LOFAR Telescope I. Foundational calibration strategy and pipeline
Creators: name: Morabito, L. K.
name: Jackson, N. J.
name: Mooney, S.
name: Sweijen, F.
name: Badole, S.
name: Kukreti, P.
name: Venkattu, D.
name: Groeneveld, C.
name: Kappes, A.
name: Bonnassieux, E.
name: Drabent, A.
name: Iacobelli, M.
name: Croston, J. H.
name: Best, P. N.
name: Bondi, M.
name: Callingham, J. R.
name: Conway, J. E.
name: Deller, A. T.
name: Hardcastle, M. J.
name: McKean, J. P.
name: Miley, G. K.
name: Moldon, J.
name: Röttgering, H. J. A.
name: Tasse, C.
name: Shimwell, T. W.
name: van Weeren, R. J.
name: Anderson, J. M.
name: Asgekar, A.
name: Avruch, I. M.
name: van Bemmel, I. M.
name: Bentum, M. J.
name: Bonafede, A.
name: Brouw, W. N.
name: Butcher, H. R.
name: Ciardi, B.
name: Corstanje, A.
name: Coolen, A.
name: Damstra, S.
name: de Gasperin, F.
name: Duscha, S.
name: Eislöffel, J.
name: Engels, D.
name: Falcke, H.
name: Garrett, M. A.
name: Griessmeier, J.
name: Gunst, A. W.
name: van Haarlem, M. P.
name: Hoeft, M.
name: van der Horst, A. J.
name: Jütte, E.
name: Kadler, M.
name: Koopmans, L. V. E.
name: Krankowski, A.
name: Mann, G.
name: Nelles, A.
name: Oonk, J. B. R.
name: Orru, E.
name: Paas, H.
name: Pandey, V. N.
name: Pizzo, R. F.
name: Pandey-Pommier, M.
name: Reich, W.
name: Rothkaehl, H.
name: Ruiter, M.
name: Schwarz, D. J.
name: Shulevski, A.
name: Soida, M.
name: Tagger, M.
name: Vocks, C.
name: Wijers, R. A. M. J.
name: Wijnholds, S. J.
name: Wucknitz, O.
name: Zarka, P.
name: Zucca, P.
Publisher: name: arXiv
Year: 2021
Subjects: subject: Instrumentation and Methods for Astrophysics (astro-ph.IM)
subject: Astrophysics of Galaxies (astro-ph.GA)
subject: FOS: Physical sciences
subject: FOS: Physical sciences

description: [abridged] The International LOFAR Telescope is an interferometer with stations spread across Europe. With baselines of up to ~2,000 km, LOFAR has the unique capability of achieving sub-arcsecond resolution at frequencies below 200 MHz, although this is technically and logistically challenging. Here we present a calibration strategy that builds on previous high-resolution work with LOFAR. 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 LOFAR Two-metre Sky Survey (LoTSS) pointing. We perform in-field delay calibration, solution referencing to other calibrators, self-calibration, and imaging of example directions of interest in the field. For this specific field and these ionospheric conditions, dispersive delay solutions can be transferred between calibrators up to ~1.5 degrees away, while phase solution transferral works well over 1 degree. We demonstrate a check of the astrometry and flux density scale. Imaging in 17 directions, the restoring beam is typically 0.3" x 0.2" although this varies slightly over the entire 5 square degree field of view. We achieve ~80 to 300 $μ$Jy/bm image rms noise, which is dependent on the distance from the phase centre; typical values are ~90 $μ$Jy/bm for the 8 hour 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 ~900 sources per LoTSS pointing. This equates 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 (LoTSS-HR) makes this estimate a lower limit.
description: Accepted to a special issue of A&amp;A on sub-arcsecond imaging with LOFAR. 24 pages, 16 figures