  We present an XMM observation of the radio jet and diffuse halo of the nearby
radio galaxy NGC6251. The EPIC spectrum of the galaxy's halo is best-fitted by
a thermal model with temperature kT~1.6 keV and subsolar abundances.
Interestingly, an additional hard X-ray component is required to fit the EPIC
spectra of the halo above 3 keV, 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 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, 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
\~1.2. 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 (delta~10).
Alternatively, weaker beaming is possible for magnetic fields several orders of
magnitude lower than the equipartition field.
