  The lobes of radio galaxies and quasars, fed by jets and hotspots, represent
a significant, and currently ill-constrained, source of energy input into the
inter-galactic medium (IGM). How much energy is input into the IGM depends on
the minimum energy to which the power-law distribution of relativistic
particles is accelerated in the hotspots. This has hitherto been unknown to
within three orders of magnitude. We present direct evidence for the discovery
of this low-energy cutoff in the lobe of a Mpc-sized radio galaxy via the
existence of extended X-ray emission, inverse-Compton scattered from aged radio
plasma, and its separation by 80kpc from regions containing freshly accelerated
plasma from the hotspot. The low-energy cutoff of gamma ~ 10^4 in the hotspot
is higher than previously thought, but reconciles discrepancies with magnetic
field estimates which had been systematically lower than equipartition values.
The inverse Compton scattering of the spent synchrotron plasma is at the
expense of cosmic microwave background (CMB) photons; we comment on the
importance of such giant radio galaxies as contaminants of CMB anisotropies.
