2007ApJ...668..203M

A new XMM-Newton observation has made possible a detailed study of both lobes of the radio galaxy Pictor A. Their X-ray emission is of nonthermal origin and due to inverse Compton scattering of cosmic microwave background (CMB) photons by relativistic electrons in the lobes, as previously found. In both lobes, the equipartition magnetic field (B_eq) is bigger than the inverse Compton value (B_IC), calculated from the radio and X-ray flux ratio. The B_eq/B_IC ratio never gets below 2, in spite of the large number of reasonable assumptions tested to calculate B_eq, suggesting a lobe energetics dominated by particles. The X-ray data quality is good enough to allow a spatially resolved analysis. Our study shows that B_IC varies throughout the lobes. It appears to increase behind the hot spots. On the contrary, a rather uniform distribution of the particles is observed. As a consequence, the radio flux density variation along the lobes appears to be mainly driven by magnetic field changes.