Detection of X-Ray Emission from the Eastern Radio Lobe of Pictor A

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 (kT=5⁺⁹_-2 keV) model and a power law with an energy index α_X=0.6+/-0.2. The X-ray emission could be thermal and produced by circumgalactic gas shocked by the expanding radio lobe or, alternatively, by inverse Compton (IC) scattering 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-averaged synchrotron radio index (<α_radio>=0.8) and the X-ray energy slope α_X. However, if this is the case, the magnetic field (B_IC~1-2 μG), as deduced from the comparison of the IC X-ray and radio fluxes, is more than a factor of 2 below the equipartition value estimated in the same X-ray region.