Discovery of Inverse-Compton X-Rays in Radio Lobes

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. 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 correspondence 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 due to an entrained thermal plasma. The detected level of 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 provides the first direct estimate of the magnetic field of a radio lobe, BIC 2-3 μG, indicating that the integrated energy output of this active galactic nucleus is near the minimum energy level.