Sub-arcsecond imaging with the International LOFAR Telescope

L K Morabito ,M Hoeft ,R J Van Weeren ,A H W M Coolen ,W Reich ,Andrzej Krankowski ,Christian Groeneveld ,D Venkattu ,F De Gasperin ,Shruti Badole ,B Ciardi ,V N Pandey ,L V E Koopmans ,W N Brouw ,H Paas ,T W Shimwell ,M A Garrett ,A Shulevski ,A Nelles ,G K Miley ,M Iacobelli ,F Sweijen ,C Vocks ,M P Van Haarlem ,Adam Deller ,M Bondì ,E Orrú ,M Pandey-Pommier ,Philippe Zarka ,S Mooney ,Judith Croston ,A Bonafede ,A Corstanje ,A W Gunst ,Hanna Rothkaehl ,P N Best ,I M Avruch ,J B R Oonk ,S Duscha ,A J Van Der Horst ,J Moldón ,D Engels ,C Tasse ,I M Van Bemmel ,O Wucknitz ,R A M J Wijers ,J R Callingham ,Jean-Mathias Grießmeier ,E Jütte ,G Mann ,S Damstra ,M Tagger ,Mark Bentum ,E Bonnassieux ,J M Anderson ,J Conway ,A Kappes ,J Eislöffel ,M Kadler ,A Drabent ,A Asgekar ,Dominik J Schwarz ,H J A Röttgering ,M Soida ,Stefan J Wijnholds ,Neal Jackson ,H R Butcher ,M Ruiter ,J P Mckean ,H Falcke ,M J Hardcastle ,Pranav Kukreti ,R Pizzo ,Pietro Zucca

doi:10.1051/0004-6361/202140649

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 deg2field of view. We find we can achieve ~80–300 μJy bm−1image rms noise, which is dependent on the distance from the phase centre; typical values are ~90 μJy bm−1for 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.

Highlights

The LOw-Frequency Array (LOFAR; van Haarlem et al 2013) is an interferometer that operates at frequencies between 10 and 240 MHz
We have developed a calibration strategy and built a pipeline to carry out this strategy, which forms the basis of high-resolution imaging with LOFAR
The International LOFAR Telescope resembles a connected-element interferometer in that data transport and correlation occurs in real time, the baseline length and independent frequency standards at the international stations necessitate a treatment that closely follows that used for Very Long Baseline Interferometry (VLBI) at centimetre and millimetre wavelengths. We developed this calibration strategy based on VLBI principles, with updates for LOFAR-specific challenges using native LOFAR software wherever possible

Summary

Introduction

The LOw-Frequency Array (LOFAR; van Haarlem et al 2013) is an interferometer that operates at frequencies between 10 and 240 MHz. Phases on longer baselines vary more rapidly than on short baselines This variation can happen in both time and frequency, meaning that small solution intervals are necessary to track the changes. Real astrophysical sources have spatial structure, and in the VLBI regime a large fraction of the emission can be resolved out, leaving only a small fraction of the signal in the compact, unresolved regions required for robust calibration This drives the need to increase the S/N and the solution intervals. Another important technique we use in high-resolution imaging with LOFAR is combining the core stations into a ‘super’ station This in effect provides a super station (ST001) in the centre of the array that is Ncore times more sensitive, providing an anchor for calibrating the international stations. The combination has to be done after any phase-rotating to different directions in the field of view (see Section 4.3)

Objectives

Results

Conclusion