Sub-arcsecond imaging with the International LOFAR Telescope I. Foundational calibration strategy and pipeline
Abstract
[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 Jy/bm image rms noise, which is dependent on the distance from the phase centre; typical values are ~90 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.
Cite
@article{arxiv.2108.07283,
title = {Sub-arcsecond imaging with the International LOFAR Telescope I. Foundational calibration strategy and pipeline},
author = {L. K. Morabito and N. J. Jackson and S. Mooney and F. Sweijen and S. Badole and P. Kukreti and D. Venkattu and C. Groeneveld and A. Kappes and E. Bonnassieux and A. Drabent and M. Iacobelli and J. H. Croston and P. N. Best and M. Bondi and J. R. Callingham and J. E. Conway and A. T. Deller and M. J. Hardcastle and J. P. McKean and G. K. Miley and J. Moldon and H. J. A. Röttgering and C. Tasse and T. W. Shimwell and R. J. van Weeren and J. M. Anderson and A. Asgekar and I. M. Avruch and I. M. van Bemmel and M. J. Bentum and A. Bonafede and W. N. Brouw and H. R. Butcher and B. Ciardi and A. Corstanje and A. Coolen and S. Damstra and F. de Gasperin and S. Duscha and J. Eislöffel and D. Engels and H. Falcke and M. A. Garrett and J. Griessmeier and A. W. Gunst and M. P. van Haarlem and M. Hoeft and A. J. van der Horst and E. Jütte and M. Kadler and L. V. E. Koopmans and A. Krankowski and G. Mann and A. Nelles and J. B. R. Oonk and E. Orru and H. Paas and V. N. Pandey and R. F. Pizzo and M. Pandey-Pommier and W. Reich and H. Rothkaehl and M. Ruiter and D. J. Schwarz and A. Shulevski and M. Soida and M. Tagger and C. Vocks and R. A. M. J. Wijers and S. J. Wijnholds and O. Wucknitz and P. Zarka and P. Zucca},
journal= {arXiv preprint arXiv:2108.07283},
year = {2022}
}
Comments
Accepted to a special issue of A&A on sub-arcsecond imaging with LOFAR. 24 pages, 16 figures