Volume 635, March 2020
|Number of page(s)||7|
|Published online||26 March 2020|
Cassiopeia A, Cygnus A, Taurus A, and Virgo A at ultra-low radio frequencies⋆
Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
2 Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
3 GRAPPA, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
4 SRON, Netherlands Institute for Space Research, Utrecht, The Netherlands
5 Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
6 ASTRON – the Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
7 Eindhoven University of Technology, PO Box 513, 5600 MB Eindhoven, The Netherlands
8 Leiden Observatory, Leiden University, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands
9 Department of Physics & Electronics, Rhodes University, 6139 Grahamstown, South Africa
10 South African Radio Astronomy Observatory, 7925 Observatory, Cape Town, South Africa
11 Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
12 Astronomical Observatory, Jagiellonian University, ul. Orla 171, 30-244 Kraków, Poland
13 LPC2E – Université d’Orléans/CNRS, Orléans, France
14 Dipartimento di Fisica e Astronomia, Universitá di Bologna, Via P. Gobetti 93/2, 40129 Bologna, Italy
15 INAF – Istituto di Radioastronomia, Bologna Via Gobetti 101, 40129 Bologna, Italy
16 LESIA & USN, Observatoire de Paris, CNRS, PSL, SU/UP/UO, 92195 Meudon, France
17 Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
18 Jodrell Bank Centre for Astrophysics, Department of Physics & Astronomy, The University of Manchester, Alan Turing Building, Oxford Road, Manchester M13 9PL, UK
19 Department of Physics, The George Washington University, 725 21st Street NW, Washington, DC 20052, USA
20 Astronomy, Physics, and Statistics Institute of Sciences (APSIS), The George Washington University, Washington, DC 20052, USA
21 Erlangen Center for Astroparticle Physics (ECAP), Friedrich-Alexander-Universität Erlangen-Nürnberg, 91058 Erlangen, Germany
22 DESY, Platanenallee 6, 15738 Zeuthen, Germany
23 Space Radio-Diagnostics Research Centre, University of Warmia and Mazury in Olsztyn, Prawochenskiego 9, 10-720 Olsztyn, Poland
24 Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
25 CSIRO Astronomy and Space Science, PO Box 1130, Bentley, WA 6102, Australia
26 Research School of Astronomy and Astrophysics, Mount Stromlo Observatory, Cotter Road, Weston Creek, ACT 2611, Australia
27 Shell Technology Center, Bangalore 562149, India
28 Max Planck Institute for Astrophysics, Karl-Schwarzschild-Str. 1, 85748 Garching, Germany
29 Fakultät für Physik, Universität Bielefeld, Postfach 100131, 33501 Bielefeld, Germany
30 USN, Station de Radioastronomie de Nançay Observatoire de Paris route de Souesmes, 18330 Nançay, France
31 Univ. Lyon, Univ. Lyon1, Ens de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon UMR5574, 9 Av Charles André, 69230 Saint-Genis-Laval, France
32 Department of Astrophysics/IMAPP, Radboud Universiteit, PO Box 9010, 6500 GL Nijmegen, The Netherlands
33 Nikhef, Science Park 105, 1098 XG Amsterdam, The Netherlands
34 Vrije Universiteit Brussel, Physics Department, Pleinlaan 2, 1050 Brussels, Belgium
35 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
36 IBCH PAS Poznan Supercomputing and Networking Center (PSNC), Poznan, Poland
37 International Centre for Radio Astronomy Research, Curtin University, GPO Box U1987, Perth, WA 6845, Australia
Accepted: 18 February 2020
Context. The four persistent radio sources in the northern sky with the highest flux density at metre wavelengths are Cassiopeia A, Cygnus A, Taurus A, and Virgo A; collectively they are called the A-team. Their flux densities at ultra-low frequencies (< 100 MHz) can reach several thousands of janskys, and they often contaminate observations of the low-frequency sky by interfering with image processing. Furthermore, these sources are foreground objects for all-sky observations hampering the study of faint signals, such as the cosmological 21 cm line from the epoch of reionisation.
Aims. We aim to produce robust models for the surface brightness emission as a function of frequency for the A-team sources at ultra-low frequencies. These models are needed for the calibration and imaging of wide-area surveys of the sky with low-frequency interferometers. This requires obtaining images at an angular resolution better than 15″ with a high dynamic range and good image fidelity.
Methods. We observed the A-team with the Low Frequency Array (LOFAR) at frequencies between 30 MHz and 77 MHz using the Low Band Antenna system. We reduced the datasets and obtained an image for each A-team source.
Results. The paper presents the best models to date for the sources Cassiopeia A, Cygnus A, Taurus A, and Virgo A between 30 MHz and 77 MHz. We were able to obtain the aimed resolution and dynamic range in all cases. Owing to its compactness and complexity, observations with the long baselines of the International LOFAR Telescope will be required to improve the source model for Cygnus A further.
Key words: radio continuum: general / techniques: interferometric
The radio models are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (18.104.22.168) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/635/A150
© ESO 2020
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