LOFAR imaging of Cygnus A – direct detection of a turnover in the hotspot radio spectra

P. Mckean (1) , H. Godfrey (1) , S. Vegetti (2) , M. Wise (3) , R. Morganti (1) , M. Hardcastle (4) , D. Rafferty (5) , J. Anderson (6) , M. Avruch (7) , R. Beck (8) , M. Bell (9, 10) , I. van Bemmel (1) , M. Bentum (1) , G. Bernardi (11) , P. Best (12) , R. Blaauw (1) , A. Bonafede (13) , F. Breitling (14) , J. Broderick (10) , M. Brüggen (13) , L. Cerrigone (1) , B. Ciardi (15) , F. de Gasperin (16) , A. Deller (1) , S. Duscha (1) , D. Engels (16) , H. Falcke (17) , R. Fallows (18) , W. Frieswijk (1) , M. Garrett (19) , Jean-Mathias Griessmeier (20, 21) , P. van Haarlem (1) , G. Heald (1) , M. Hoeft (22) , A. J. van Der Horst (3) , M. Iacobelli (23) , H. Intema (19) , E. Juette (24) , A. Karastergiou (25) , V. I. Kondratiev (1) , M. Kuniyoshi (26) , G. Kuper (1) , L. V. E. Koopmans (7) , J. van Leeuwen (3) , P. Maat (1) , G. Mann (14) , S. Markoff (27) , R. Mcfadden (1) , D. Mckay-Bukowski (28) , D. D. Mulcahy (10) , H. Munk (1) , A. Nelles (29) , E. Orru (1) , H. Paas (30) , M. Pandey-Pommier (31) , M. Pietka (32) , R. Pizzo (1) , A. Polatidis (1) , W. Reich (26) , H. J. A. Rottgering (19) , A. Rowlinson (33) , A. M. M. Scaife (34) , M. Serylak (35) , A. Shulevski (1) , J. Sluman (1) , O. Smirnov (36) , M. Steinmetz (37) , A. Stewart (38) , J. Swinbank (3) , Michel Tagger (21) , S. Thoudam (29) , C. Toribio (1) , R. Vermeulen (1) , C. Vocks (14) , R. J. van Weeren (39) , O. Wucknitz (40) , S. Yatawatta (1) , P. Zarka (41)

1 ASTRON - Netherlands Institute for Radio Astronomy
2 SRON - SRON Netherlands Institute for Space Research
3 AI PANNEKOEK - Astronomical Institute Anton Pannekoek
4 UH - University of Hertfordshire [Hatfield]
5 Department of Astronomy and Astrophysics [PennState]
6 APS - Department of Atmospheric and Planetary Sciences [Hampton]
7 Kapteyn Astronomical Institute [Groningen]
8 UT - Université de Tours
9 CSIRO Astronomy and Space Science
10 University of Southampton
11 LPNHE - Laboratoire de Physique Nucléaire et de Hautes Énergies
12 University of Edinburgh
13 Jacobs University [Bremen]
14 AIP - Leibniz-Institut für Astrophysik Potsdam
15 Max Planck Institute for Astrophysics
16 Hamburger Sternwarte/Hamburg Observatory
17 IMAPP - Institute for Mathematics, Astrophysics and Particle Physics
18 Institute of Mathematical and Physical Sciences
19 Leiden Observatory [Leiden]
20 USN - Unité Scientifique de la Station de Nançay
21 LPC2E - Laboratoire de Physique et Chimie de l'Environnement et de l'Espace
22 JIVE ERIC - Joint Institute for VLBI in Europe
23 DPMMS - Department of Pure Mathematics and Mathematical Statistics
24 Ruhr-Universität Bochum [Bochum]
25 Oxford Astrophysics
26 MPIFR - Max-Planck-Institut für Radioastronomie
27 UvA - University of Amsterdam [Amsterdam]
28 University of Oulu
29 Radboud University [Nijmegen]
30 University of Groningen, Groningen, Netherlands
31 CRAL - Centre de Recherche Astrophysique de Lyon
32 University of Oxford
33 ATNF - Australia Telescope National Facility
34 School of Physics and Astronomy [Southampton]
35 Department of Physics and Astronomy [South Africa]
36 Rhodes University, Grahamstown
37 DSMZ - Leibniz-Institut DSMZ-Deutsche Sammlung von Mikroorganismen und Zellkulturen GmbH / Leibniz Institute DSMZ-German Collection of Microorganisms and Cell Cultures
38 LASP - Laboratory for Atmospheric and Space Physics [Boulder]
39 CfA - Harvard-Smithsonian Center for Astrophysics
40 AlfA - Argelander-Institut für Astronomie
41 LESIA - Laboratoire d'études spatiales et d'instrumentation en astrophysique

The low-frequency radio spectra of the hotspots within powerful radio galaxies can provide valuable information about the physical processes operating at the site of the jet termination. These processes are responsible for the dissipation of jet kinetic energy, particle acceleration, and magnetic-field generation. Here, we report new observations of the powerful radio galaxy Cygnus A using the Low Frequency Array (LOFAR) between 109 and 183 MHz, at an angular resolution of ∼3.5 arcsec. The radio emission of the lobes is found to have a complex spectral index distribution, with a spectral steepening found towards the centre of the source. For the first time, a turnover in the radio spectrum of the two main hotspots of Cygnus A has been directly observed. By combining our LOFAR imaging with data from the Very Large Array at higher frequencies, we show that the very rapid turnover in the hotspot spectra cannot be explained by a low-energy cutoff in the electron energy distribution, as has been previously suggested. Thermal (free–free) absorption or synchrotron self-absorption models are able to describe the low-frequency spectral shape of the hotspots; however, as with previous studies, we find that the implied model parameters are unlikely, and interpreting the spectra of the hotspots remains problematic.

Keywords: Galaxies: active: individual: Cygnus A; Radio continuum: galaxies

Domains: Sciences of the Universe [physics]

P. Mckean, H. Godfrey, S. Vegetti, M. Wise, R. Morganti, et al.. LOFAR imaging of Cygnus A – direct detection of a turnover in the hotspot radio spectra. Monthly Notices of the Royal Astronomical Society, 2016, 463 (3), pp.3143 - 3150. ⟨10.1093/mnras/stw2105⟩. ⟨insu-01363888⟩