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 \sim 1.6 keV and sub-solar 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 ( 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 < < 1 \mu Gauss , 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 ( \sim 3.5 ^ { \prime } ) jet , confirming previous ROSAT findings .
Both the EPIC and ACIS spectra of the jet are best-fitted by a power law with photon index \Gamma \sim 1.2 , fixed Galactic column density , and 1 keV flux F _ { 1 ~ { } keV } = 2.1 nJy .
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 \sim 10 ) .
Alternatively , weaker beaming is possible for magnetic fields several orders of magnitude lower than the equipartition field .