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

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

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


Authors

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. McKean

G.K. Miley

J. Moldon

H.J.A. Rottgering

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. Eisloffel

D. Engels

H. Falcke

M.A. Garrett

J. Griessmeier

A.W. Gunst

M.P. van Haarlem

M. Hoeft

A.J. van der Horst

E. Jutte

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



Abstract

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.

Citation

Morabito, L., Jackson, N., Mooney, S., Sweijen, F., Badole, S., Kukreti, P., …Zucca, P. (2021). Sub-arcsecond imaging with the International LOFAR Telescope I. Foundational calibration strategy and pipeline. Astronomy & Astrophysics, 658, Article A1. https://doi.org/10.1051/0004-6361/202140649

Journal Article Type Article
Acceptance Date Apr 1, 2021
Online Publication Date Jan 25, 2021
Publication Date 2021-02
Deposit Date May 7, 2021
Publicly Available Date Mar 28, 2024
Journal Astronomy & Astrophysics
Print ISSN 0004-6361
Electronic ISSN 1432-0746
Publisher EDP Sciences
Peer Reviewed Peer Reviewed
Volume 658
Article Number A1
DOI https://doi.org/10.1051/0004-6361/202140649

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Copyright Statement
Reproduced with permission, © ESO.





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