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The low-frequency radio spectra of the hotspots within powerful radiogalaxies can provide valuable information about the physical processesoperating at the site of the jet termination. These processes are responsiblefor the dissipation of jet kinetic energy, particle acceleration, andmagnetic-field generation. Here we report new observations of the powerfulradio galaxy Cygnus A using the Low Frequency Array (LOFAR) between 109 and 183MHz, at an angular resolution of ~3.5 arcsec. The radio emission of the lobesis found to have a complex spectral index distribution, with a spectralsteepening found towards the centre of the source. For the first time, aturnover in the radio spectrum of the two main hotspots of Cygnus A has beendirectly observed. By combining our LOFAR imaging with data from the Very LargeArray at higher frequencies, we show that the very rapid turnover in thehotspot spectra cannot be explained by a low-energy cut-off in the electronenergy distribution, as has been previously suggested. Thermal (free-free)absorption or synchrotron self absorption models are able to describe thelow-frequency spectral shape of the hotspots, however, as with previousstudies, we find that the implied model parameters are unlikely, andinterpreting the spectra of the hotspots remains problematic.

LOFAR imaging of Cygnus A -- Direct detection of a turnover in the hotspot radio spectra