2015A&A...574A..30P

The jets in Faranoff-Riley type II AGN are supposed come to an abrupt halt in hotspots on opposite sides of the nucleus. Quite commonly, two hotspots are observed in each lobe. The origin of the second hotspot is currently poorly understood. Our aims are to determine the origin of the secondary hotspot in the western lobe of Cygnus A from high resolution multifrequency radio images; to determine the minimum Lorentz factor of the electrons in the hotspots, often referred to as the low-energy turnover; and to study the magnetic field configuration of the hotspots. We used 151MHz Merlin and 327MHz, 1.4, 5, 8, 15, and 43GHz VLA images to determine the centroid of the peak luminosity, the spectral shape, and polarization fraction of both hotspots in the western lobe of Cygnus A. We find a spatial shift in peak luminosity between the lower and higher frequency images for both hotspots. We determine the minimum Lorentz factor of the electrons to be ∼1000, and show that most of the emission from the primary hotspot is linearly polarized. The minimum energy magnetic field strength is found to range between ∼0.14 and ∼0.5mG in both the primary and secondary hotspots. From the low polarization and the determined outflow velocity, we conclude that the secondary hotspot is no longer a strong shock, and is an expanding, and hence a fading hotspot. This hotspot has an age that is of the same order of magnitude as the jet precession period.