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

L. K. Morabito, N. J. Jackson, S. Mooney, F. Sweijen, S. Badole, P. Kukreti, D. Venkattu, C. Groeneveld, A. Kappes, E. Bonnassieux, A. Drabent, M. Iacobelli, J. H. Croston, P. N. Best, M. Bondi, J. R. Callingham, J. E. Conway, A. T. Deller, M. J. Hardcastle, J. P. McKeanG. K. Miley, J. Moldon, H. J. A. Röttgering, C. Tasse, T. W. Shimwell, R. J. van Weeren, J. M. Anderson, A. Asgekar, I. M. Avruch, I. M. van Bemmel, M. J. Bentum, A. Bonafede, W. N. Brouw, H. R. Butcher, B. Ciardi, A. Corstanje, A. Coolen, S. Damstra, F. de Gasperin, S. Duscha, J. Eislöffel, D. Engels, H. Falcke, M. A. Garrett, J. Griessmeier, A. W. Gunst, M. P. van Haarlem, M. Hoeft, A. J. van der Horst, E. Jütte, M. Kadler, L. V. E. Koopmans, A. Krankowski, G. Mann, A. Nelles, J. B. R. Oonk, E. Orru, H. Paas, V. N. Pandey, R. F. Pizzo, M. Pandey-Pommier, W. Reich, H. Rothkaehl, M. Ruiter, D. J. Schwarz, A. Shulevski, M. Soida, M. Tagger, C. Vocks, R. A. M. J. Wijers, S. J. Wijnholds, O. Wucknitz, P. Zarka, P. Zucca

Research output: Contribution to journalArticlepeer-review

Abstract

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. However, it is technicallyand logistically challenging to process LOFAR data at this resolution. To date only a handful of publications have exploited thiscapability. Here we present a calibration strategy that builds on previous high-resolution work with LOFAR. It is implemented in apipeline 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-resolutionimaging with LOFAR, and describe the pipeline, which is publicly available, in detail. We demonstrate the calibration strategy byusing the pipeline on P205+55, a typical LoTSS pointing with an 8 hour observation and 13 international stations. We performin-field delay calibration, solution referencing to other calibrators in the field, self-calibration of these calibrators, and imaging ofexample directions of interest in the field. We find that for this specific field and these ionospheric conditions, dispersive delaysolutions can be transferred between calibrators up to∼1.5 degrees away, while phase solution transferral works well over∼1degree. We also demonstrate a check of the astrometry and flux density scale with the in-field delay calibrator source. Imaging in17 directions, we find the restoring beam is typically∼0.3′′×0.2′′although this varies slightly over the entire 5 square degree fieldof view. We find we can achieve∼80 to 300μJy bm−1image rms noise, which is dependent on the distance from the phase centre;typical values are∼90μJy bm−1for the 8 hour observation with 48 MHz of bandwidth. Seventy percent of processed sourcesare detected, and from this we estimate that we should be able to image roughly 900 sources per LoTSS pointing. This equatesto∼3 million sources in the northern sky, which LoTSS will entirely cover in the next several years. Future optimisation of thecalibration strategy for efficient post-processing of LoTSS at high resolution (LoTSS-HR) makes this estimate a lower limit.
Original languageEnglish
JournalAstronomy and Astrophysics
DOIs
Publication statusAccepted/In press - 1 Apr 2021

Keywords

  • astro-ph.IM
  • astro-ph.GA

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