2004A&A...414..885S

We present an XMM-Newton observation of the radio jet and diffuse halo of the nearby radio galaxy NGC 6251. The EPIC spectrum of the galaxy's halo is best-fitted by a thermal model with temperature kT∼1.6keV and sub-solar abundances. Interestingly, an additional hard X-ray component is required to fit the EPIC spectra of the halo above 3keV, and is independently confirmed by an archival Chandra observation. However, its physical origin is not clear. Contribution from a population of undetected Low Mass X-ray Binaries seems unlikely. Instead, the hard X-ray component could be due to inverse Compton scattering of the CMB photons (IC/CMB) off relativistic electrons scattered throughout the halo of the galaxy, or non-thermal bremsstrahlung emission. The IC/CMB interpretation, together with limits on the diffuse radio emission, implies a very weak magnetic field, B≪1µGauss, while a non-thermal bremsstrahlung origin implies the presence of a large number of very energetic electrons. We also detect X-ray emission from the outer (∼3.5') jet, confirming previous ROSAT findings. Both the EPIC and ACIS spectra of the jet are best-fitted by a power law with photon index Γ∼1.2, fixed Galactic column density, and 1keV flux F_1keV=2.1nJy. A thermal model is formally ruled out by the data. Assuming an origin of the X-rays from the jet via IC/CMB, as suggested by energetic arguments, and assuming equipartition implies a large Doppler factor (δ∼10). Alternatively, weaker beaming is possible for magnetic fields several orders of magnitude lower than the equipartition field.