Sub-arcsecond imaging with the International LOFAR Telescope: I. Foundational calibration strategy and pipeline
Journal article, 2022
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.
Techniques: high angular resolution
Radiation mechanisms: non-thermal
Galaxies: active
Galaxies: jets
Author
L. Morabito
Durham University
N. J. Jackson
University of Manchester
S. Mooney
University College Dublin
F. Sweijen
Leiden University
S. Badole
University of Manchester
P. Kukreti
Netherlands Institute for Radio Astronomy (ASTRON)
University of Groningen
D. Venkattu
AlbaNova University Center
C. Groeneveld
Leiden University
A. Kappes
University of Würzburg
E. Bonnassieux
University of Bologna
A. Drabent
Thüringer Landessternwarte Tautenburg
M. Iacobelli
Netherlands Institute for Radio Astronomy (ASTRON)
J. H. Croston
Open University
P. N. Best
University of Edinburgh
M. Bondi
Istituto di Radioastronomia
J. R. Callingham
Netherlands Institute for Radio Astronomy (ASTRON)
Leiden University
John Conway
Chalmers, Space, Earth and Environment, Onsala Space Observatory
A. Deller
Swinburne University of Technology
M. J. Hardcastle
University of Hertfordshire
J. McKean
University of Groningen
Netherlands Institute for Radio Astronomy (ASTRON)
G. K. Miley
Leiden University
J. Moldon
Spanish National Research Council (CSIC)
H. J.A. Röttgering
Leiden University
C. Tasse
Rhodes University
Observatoire de Paris-Meudon
T. W. Shimwell
Netherlands Institute for Radio Astronomy (ASTRON)
Leiden University
R. van Weeren
Leiden University
J. M. Anderson
German Research Centre for Geosciences (GFZ)
Technische Universität Berlin
A. Asgekar
Shell Technology Center
Netherlands Institute for Radio Astronomy (ASTRON)
I.M. Avruch
Sci & Technol BV
Netherlands Institute for Radio Astronomy (ASTRON)
I. van Bemmel
Joint Institute for VLBI in Europe (JIVE)
M. J. Bentum
Netherlands Institute for Radio Astronomy (ASTRON)
Eindhoven University of Technology
A. Bonafede
University of Bologna
University of Hamburg
Istituto di Radioastronomia
W. N. Brouw
University of Groningen
H. R. Butcher
Australian National University
B. Ciardi
Max Planck Society
A. Corstanje
Radboud University
Vrije Universiteit Brussel (VUB)
A. H.W.M. Coolen
Netherlands Institute for Radio Astronomy (ASTRON)
Sieds Damstra
Netherlands Institute for Radio Astronomy (ASTRON)
F. De Gasperin
Istituto di Radioastronomia
University of Hamburg
S. Duscha
Netherlands Institute for Radio Astronomy (ASTRON)
J. Eisloffel
Thüringer Landessternwarte Tautenburg
D. Engels
University of Hamburg
H.D. Falcke
Radboud University
M. A. Garrett
Leiden University
University of Manchester
J.M. Grießmeier
University of Orléans
Station de Radioastronomie de Nançay
A.W. Gunst
Netherlands Institute for Radio Astronomy (ASTRON)
M. P. van Haarlem
Netherlands Institute for Radio Astronomy (ASTRON)
M. Hoeft
Thüringer Landessternwarte Tautenburg
A. J. van der Horst
George Washington University
E. Jutte
Ruhr-Universität Bochum
M. Kadler
University of Würzburg
L.V.E. Koopmans
University of Groningen
A. Krankowski
University of Warmia and Mazury in Olsztyn
G. Mann
Leibniz Institute for Astrophysics Potsdam
A. Nelles
University of Erlangen-Nuremberg (FAU)
Deutsches Elektronen-Synchrotron (DESY)
J. B. R. Oonk
Surfsara Bv
E. Orrú
Netherlands Institute for Radio Astronomy (ASTRON)
H. Paas
University of Groningen
V. N. Pandey
Netherlands Institute for Radio Astronomy (ASTRON)
R. Pizzo
Netherlands Institute for Radio Astronomy (ASTRON)
M. Pandey-Pommier
Université de Lyon
W. Reich
Max Planck Society
Hanna Rothkaehl
Polish Academy of Sciences
Mark Ruiter
Netherlands Institute for Radio Astronomy (ASTRON)
D. J. Schwarz
Bielefeld University
A. Shulevski
Leiden University
University of Amsterdam
M. Soida
Jagiellonian University in Kraków
M. Tagger
University of Orléans
C. Vocks
Leibniz Institute for Astrophysics Potsdam
Ramj Wijers
University of Amsterdam
S. J. Wijnholds
Netherlands Institute for Radio Astronomy (ASTRON)
O. Wucknitz
Max Planck Society
P.M. Zarka
Station de Radioastronomie de Nançay
LESIA - Laboratoire d'Etudes Spatiales et d'Instrumentation en Astrophysique
P. Zucca
Netherlands Institute for Radio Astronomy (ASTRON)
Astronomy and Astrophysics
0004-6361 (ISSN) 1432-0746 (eISSN)
Vol. 658 A1Subject Categories (SSIF 2011)
Other Physics Topics
Signal Processing
Computer Vision and Robotics (Autonomous Systems)
DOI
10.1051/0004-6361/202140649