First LOFAR observations at very low frequencies of cluster-scale non-thermal emission: the case of Abell 2256

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: rvweeren@strw.leidenuniv.nl
² Netherlands Institute for Radio Astronomy (ASTRON), Postbus 2, 7990 AA Dwingeloo, The Netherlands
³ Jacobs University Bremen, Campus Ring 1, 28759 Bremen, Germany
⁴ INAF/Istituto di Radioastronomia, via Gobetti 101, 40129 Bologna, Italy
⁵ Laboratoire Lagrange, UMR 7293, Université de Nice Sophia-Antipolis, CNRS, Observatoire de la Côte d’Azur, 06300 Nice, France
⁶ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
⁷ GEPI, Observatoire de Paris-Meudon, 5 place Jules Janssen, 92190 Meudon, France
⁸ Max Planck Institute for Astrophysics, Karl Schwarzschild Str. 1, 85741 Garching, Germany
⁹ Kapteyn Astronomical Institute, PO Box 800, 9700 AV Groningen, The Netherlands
¹⁰ Jodrell Bank Center for Astrophysics, School of Physics and Astronomy, The University of Manchester, Manchester M13 9PL, UK
¹¹ Onsala Space Observatory, Dept. of Earth and Space Sciences, Chalmers University of Technology, 43992 Onsala, Sweden
¹² National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475, USA
¹³ NavalResearch Laboratory, 4555 Overlook Avenue SW, Washington D. C. 20375, USA
¹⁴ Department of Physics & Astronomy, The Open University, UK
¹⁵ Space Science Department, Rutherford Appleton Laboratory, Chilton, UK
¹⁶ Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
¹⁷ Astronomical Institute “Anton Pannekoek”, University of Amsterdam, Postbus 94249, 1090 GE Amsterdam, The Netherlands
¹⁸ Argelander-Institut für Astronomie, University of Bonn, Auf dem Hügel 71, 53121 Bonn, Germany
¹⁹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
²⁰ School of Physics and Astronomy, University of Southampton, Southampton, SO17 1BJ, UK
²¹ Harvard-Smithsonian Center for Astrophysics, Garden Street 60, Cambridge, MA, 02138, USA
²² Institute for Astronomy, University of Edinburgh, Royal Observatory of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
²³ Mt Stromlo Obs., Research School of Astronomy and Astrophysics, Australian National University, Weston, A.C.T. 2611, Australia
²⁴ Centre de Recherche Astrophysique de Lyon, Observatoire de Lyon, 9 Av. Charles André, 69561 Saint Genis Laval Cedex, France
²⁵ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
²⁶ Laboratoire de Physique et Chimie de l’Environnement et de l’Espace 3A, Avenue de la Recherche Scientifique, 45071 Orléans Cedex 2, France

Received: 2 March 2012
Accepted: 12 April 2012

Abstract

Abell 2256 is one of the best known examples of a galaxy cluster hosting large-scale diffuse radio emission that is unrelated to individual galaxies. It contains both a giant radio halo and a relic, as well as a number of head-tail sources and smaller diffuse steep-spectrum radio sources. The origin of radio halos and relics is still being debated, but over the last years it has become clear that the presence of these radio sources is closely related to galaxy cluster merger events. Here we present the results from the first LOFAR low band antenna (LBA) observations of Abell 2256 between 18 and 67 MHz. To our knowledge, the image presented in this paper at 63 MHz is the deepest ever obtained at frequencies below 100 MHz in general. Both the radio halo and the giant relic are detected in the image at 63 MHz, and the diffuse radio emission remains visible at frequencies as low as 20 MHz. The observations confirm the presence of a previously claimed ultra-steep spectrum source to the west of the cluster center with a spectral index of  −2.3 ± 0.4 between 63 and 153 MHz. The steep spectrum suggests that this source is an old part of a head-tail radio source in the cluster. For the radio relic we find an integrated spectral index of  −0.81 ± 0.03, after removing the flux contribution from the other sources. This is relatively flat which could indicate that the efficiency of particle acceleration at the shock substantially changed in the last  ~0.1 Gyr due to an increase of the shock Mach number. In an alternative scenario, particles are re-accelerated by some mechanism in the downstream region of the shock, resulting in the relatively flat integrated radio spectrum. In the radio halo region we find indications of low-frequency spectral steepening which may suggest that relativistic particles are accelerated in a rather inhomogeneous turbulent region.