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 4 M. Hardcastle 5 D. Rafferty 6 J. Anderson 7 M. Avruch 8 R. Beck 9 M. Bell 10, 11 I. van Bemmel 4 M. Bentum 1 G. Bernardi 12 P. Best 13 R. Blaauw 4 A. Bonafede 14 F. Breitling 15 J. Broderick 11 M. Brüggen 14 L. Cerrigone 1 B. Ciardi 16 F. de Gasperin 17 A. Deller 4 S. Duscha 1 D. Engels 17 H. Falcke 18 R. Fallows 19 W. Frieswijk 4 M. Garrett 20 Jean-Mathias Grießmeier 21, 22 P. van Haarlem 4 G. Heald 4 M. Hoeft 23 A. J. van der Horst 3 M. Iacobelli 24 H. Intema 20 E. Juette 25 A. Karastergiou 26 V. I. Kondratiev 1 M. Kuniyoshi 27 G. Kuper 4 L. V. E. Koopmans 8 J. van Leeuwen 3 P. Maat 1 G. Mann 28 S. Markoff 29 R. Mcfadden 1 D. Mckay-Bukowski 30 D. D. Mulcahy 11 H. Munk 1 A. Nelles 31 E. Orru 4 H. Paas 32 M. Pandey-Pommier 33 M. Pietka 34 R. Pizzo 4 A. Polatidis 4 W. Reich 27 H. J. A. Rottgering 20 A. Rowlinson 35 A. M. M. Scaife 36 M. Serylak 37 A. Shulevski 4 J. Sluman 4 O. Smirnov 38 M. Steinmetz 39 A. Stewart 40 J. Swinbank 3 Michel Tagger 22 S. Thoudam 31 C. Toribio 4 R. Vermeulen 1 C. Vocks 28 R. J. van Weeren 41 O. Wucknitz 42 S. Yatawatta 4 P. Zarka 43
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.
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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, Oxford University Press (OUP): Policy P - Oxford Open Option A, 2016, 463 (3), pp.3143 - 3150. ⟨10.1093/mnras/stw2105⟩. ⟨insu-01363888⟩

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