We observed the radio galaxy PKS 1343 -601 (Centaurus B) in X-rays with ASCA and discovered diffuse hard X-ray emission from its radio lobe region. The obtained flux and spectrum strongly suggest that the X-rays are produced via an inverse-Compton (IC) process, in which the cosmic microwave back ground photons are boosted up to X-rays by synchrotron electrons. This is the second case, following the case of Fornax A, of the detection of the IC X-rays from radio lobes. The observed X-ray emission is brighter than expected in the case of minimum energy density, indicating particle-dominated lobes. Com parison of the X-ray and radio images reveals an outward increase in the magnetic energy density. This infers a gradual compression of magnetic fields toward the edge of the lobes.
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