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

Morabito, L. K. and Jackson, N. J. and Mooney, S. and Sweijen, F. and Badole, S. and Kukreti, P. and Venkattu, D. and Groeneveld, C. and Kappes, A. and Bonnassieux, E. and Drabent, A. and Iacobelli, M. and Croston, J. H. and Best, P. N. and Bondi, M. and Callingham, J. R. and Conway, J. E. and Deller, A. T. and Hardcastle, M. J. and McKean, J. P. and Miley, G. K. and Moldon, J. and Rottgering, H. J. A. and Tasse, C. and Shimwell, T. W. and van Weeren, R. J. and Anderson, J. M. and Asgekar, A. and Avruch, I. M. and van Bemmel, I. M. and Bentum, M. J. and Bonafede, A. and Brouw, W. N. and Butcher, H. R. and Ciardi, B. and Corstanje, A. and Coolen, A. and Damstra, S. and de Gasperin, F. and Duscha, S. and Eisloffel, J. and Engels, D. and Falcke, H. and Garrett, M. A. and Griessmeier, J. and Gunst, A. W. and van Haarlem, M. P. and Hoeft, M. and van der Horst, A. J. and Jutte, E. and Kadler, M. and Koopmans, L. V. E. and Krankowski, A. and Mann, G. and Nelles, A. and Oonk, J. B. R. and Orru, E. and Paas, H. and Pandey, V. N. and Pizzo, R. F. and Pandey-Pommier, M. and Reich, W. and Rothkaehl, H. and Ruiter, M. and Schwarz, D. J. and Shulevski, A. and Soida, M. and Tagger, M. and Vocks, C. and Wijers, R. A. M. J. and Wijnholds, S. J. and Wucknitz, O. and Zarka, P. and Zucca, P. (2021) 'Sub-arcsecond imaging with the International LOFAR Telescope I. Foundational calibration strategy and pipeline.', Astronomy & Astrophysics, 658 . A1.


The International LOFAR Telescope is an interferometer with stations spread across Europe. With baselines of up 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 ~1.5° away, while phase solution transferral works well over ~1°. 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 ~0.3′′ ×0.2′′ although this varies slightly over the entire 5 deg2 field of view. We find we can achieve ~80–300 μJy bm−1 image rms noise, which is dependent on the distance from the phase centre; typical values are ~90 μJy 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 ~ 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.

Item Type:Article
Full text:Publisher-imposed embargo
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Publisher statement:Reproduced with permission, © ESO.
Date accepted:01 April 2021
Date deposited:30 June 2021
Date of first online publication:25 January 2021
Date first made open access:30 June 2021

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