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Inverse Compton (IC) scattering of cosmic microwave background photons into the X-ray band by relativistic electrons in diffuse radio lobes is an obligatory process. Yet, despite searches for 25 years, no clear example has been found. We present the strongest evidence to date for the detection of these IC X-rays. They appear in a deep image of the nearby radio galaxy Fornax A (= NGC 1316) obtained with the Position Sensitive Proportional Counter on board the ROSAT satellite. The spatial correspondance of the X-ray and radio emission is excellent. The absence of Faraday depolarization in the lobes argues strongly against the possibility that the X-rays are thermal in origin. The detected level of IC X-rays is somewhat higher than, but consistent with, that expected from the assumption of equipartition between magnetic fields and relativistic electrons in the lobes. This observation establishes the magnetic field of a radio lobe for the first time, $B_{IC} \simeq 2$ $\mu$G, and demonstrates that the integrated energy output of this active galactic nucleus is near the minimum energy level.
