  Radio relics are Mpc-scale diffuse radio sources at the peripheries of galaxy
clusters which are thought to trace outgoing merger shocks. We present
XMM-Newton and Suzaku observations of the galaxy cluster Abell 2744 (z=0.306),
which reveal the presence of a shock front 1.5 Mpc East of the cluster core.
The surface-brightness jump coincides with the position of a known radio relic.
Although the surface-brightness jump indicates a weak shock with a Mach number
$\mathcal{M}=1.7_{-0.3}^{+0.5}$, the plasma in the post-shock region has been
heated to a very high temperature ($\sim13$ keV) by the passage of the shock
wave. The low acceleration efficiency expected from such a weak shock suggests
that mildly relativistic electrons have been re-accelerated by the passage of
the shock front.
