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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 [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

Abstract : 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

Document type :

Journal articles

Domain :

Sciences of the Universe [physics]

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Submitted on : Tuesday, May 16, 2017 - 10:46:06 AM
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