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M.J. Hardcastle, M. Birkinshaw, D.M. Worrall (Department of Physics, University of Bristol)
Measurements of the magnetic field strength in components of extragalactic radio sources are vital for an understanding of their energetics and dynamics. X-ray observations of the faint inverse-Compton emission from radio components are the only source of such data. The bright `hotspots' of powerful double (FRII) radio sources are the best places for inverse-Compton studies, and are of particular importance as the dominant sites of particle acceleration. Before Chandra, only Cygnus A was bright enough for this inverse-Compton emission to be detected, but with Chandra we can now detect inverse-Compton emission from more sources and make the first X-ray maps of hotspots. Chandra has already detected X-ray emission at the predicted level from 3C\,295. Here we report on a new Chandra hotspot detection in the z=0.2177 radio galaxy 3C\,123. The hotspot has a 1-keV flux density ~4 nJy, and is slightly resolved. We show that the flux density is close to the value predicted by accurate models of inverse-Compton emission from the synchrotron-emitting electron population assuming equipartition between electron and magnetic field energy densities, and that the spatial structure and spectrum of the hotspot are also consistent with this model. 3C\,123 is thus the third radio galaxy to show direct evidence for equipartition in the hotspots. We briefly compare 3C\,123 with other detected X-ray hotspots, and emphasise the need for further Chandra data to assess whether `typical' radio galaxies follow the pattern of Cygnus A, 3C\,295 and 3C\,123, and conclude by discussing the implications for equipartition in the other components of radio sources.
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