Pushing sub-arcsecond resolution imaging down to 30 MHz with the trans-European International LOFAR Telescope

¹ Leiden Observatory, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands
e-mail: groeneveld@strw.leidenuniv.nl
² Centre for Extragalactic Astronomy, Department of Physics, Durham University, Durham DH1 3LE, UK
³ Institute for Computational Cosmology, Department of Physics, University of Durham, South Road, Durham DH1 3LE, UK
⁴ Hamburger Sternwarte, University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
⁵ INAF – Istituto di Radioastronomia, via P. Gobetti 101, 40129, Bologna, Italy
⁶ ASTRON, Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, Dwingeloo, 7991 PD, The Netherlands
⁷ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
⁸ Instituto de Astrofísica de Andalucía (IAA, CSIC), Glorieta de las Astronomía, s/n, 18008 Granada, Spain
⁹ Institut für Theoretische Physik und Astrophysik, Universität Würzburg, Emil-Fischer-Str. 31, 97074 Würzburg, Germany

Received: 20 May 2021
Accepted: 28 September 2021

Abstract

Relatively little information is available about the Universe at ultra-low radio frequencies (ULF; i.e., below 50 MHz), although the ULF spectral window contains a wealth of unique diagnostics for studying galactic and extragalactic phenomena. Subarcsecond resolution imaging at these frequencies is extremely difficult, due to the long baselines (>1000 km) required and large ionospheric perturbations. We have conducted a pilot project to investigate the ULF performance and potential of the International LOFAR Telescope (ILT), a trans-European interferometric array with baselines up to ~2000 km and observing frequencies down to 10 MHz. We have successfully produced images with subarcsecond resolution for six radio sources at frequencies down to 30 MHz. This resolution is more than an order of magnitude better than pre-ILT observations at similar frequencies. The six targets that we imaged (3C 196, 3C 225, 3C 273, 3C 295, 3C 298, and 3C 380) are bright radio sources with compact structures. By comparing our data of 3C 196 and 3C 273 with observations at higher frequencies, we investigate their spatially resolved radio spectral properties. Our success shows that at frequencies down to 30 MHz, subarcsecond imaging with the ILT is possible. Further analysis is needed to determine the feasibility of observations of fainter sources or sources with less compact emission.