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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 
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, 2016, 463 (3), pp.3143 - 3150. ⟨10.1093/mnras/stw2105⟩. ⟨insu-01363888⟩



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