Magnetic field strengths in the hotspots and lobes of three powerful FRII radio sources

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Abstract
We have made deep Chandra observations of three powerful FRII radio sources: two quasars (3C263 and 3C351) and one radio galaxy (3C330). X-ray emission from hotspots and lobes, as well as from the active nucleus, is detected in each source. We model the hotspots' synchrotron spectra using VLA, BIMA and HST data. In 3C263 and 3C330, the hotspots' X-ray emission is at a level consistent with being synchrotron self-Compton (SSC) emission, with a hotspot magnetic field close to the equipartition value. In the two hotspots of 3C351, however, an SSC origin for the X-rays would require the magnetic field strength to be an order of magnitude below the equipartition value in our models: in addition, there are offsets between the radio, optical and X-ray emission from the secondary hotspot which are hard to explain in a simple SSC model. We discuss the emission mechanisms that may be responsible for these observations. On our preferred model, the X-ray emission from the radio lobes of the three sources is due to inverse-Compton scattering of the microwave background radiation. If this is the case, the magnetic field strengths in the lobes are typically about a factor 2 below the equipartition values, assuming uniform lobe electron and magnetic field distributions. We detect extended X-ray emission, which we attribute to a cluster/group environment, around 3C263 and 3C330. This detection allows us to show that the lobes are close to pressure balance with their surroundings, as long as no non-radiating particles contribute to the internal pressure of the lobes.

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