  The XMM-Newton satellite has revealed extended X-ray emission from the
eastern radio lobe of the Fanaroff-Riley II Radio Galaxy Pictor A. The X-ray
spectrum, accumulated on a region covering about half the entire radio lobe, is
well described by both a thermal model and a power law. The X-ray emission
could be thermal and produced by circum-galactic gas shocked by the expanding
radio lobe or, alternatively, by Inverse Compton (IC) of cosmic microwave
background photons by relativistic electrons in the lobe. The latter
possibility seems to be supported by the good agreement between the
lobe-average synchrotron radio index and the X-ray energy slope. However, if
this is the case, the magnetic field, as deduced from the comparison of the IC
X-ray and radio fluxes, is more than a factor 2 below the equipartition value
estimated in the same X-ray region.
