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Published online by Cambridge University Press: 25 May 2016
Fornax A (NGC 1316) is a radio galaxy with prototypical double lobes. Feigelson et al. (1995 ApJ 449, L149) and Kaneda et al. (1995 ApJ 454, L13) detected inverse Compton X-rays for the first time from its radio lobes, and unambiguously derived the lobe magnetic field intensity of 2-4 μG. Accordingly, the radio-emitting electrons in the lobes are inferred to have a Lorentz factor of 104, and hence a synchrotron life time of ~108 yr. This means that Fornax A was highly active at least 108 years ago.