  Galaxy clusters form through a sequence of mergers of smaller galaxy clusters
and groups. Models of diffusive shock acceleration (DSA) suggest that in shocks
that occur during cluster mergers, particles are accelerated to relativistic
energies, similar to supernova remnants. Together with magnetic fields these
particles emit synchrotron radiation and may form so-called radio relics. Here
we report the detection of a radio relic for which we find highly aligned
magnetic fields, a strong spectral index gradient, and a narrow relic width,
giving a measure of the magnetic field in an unexplored site of the universe.
Our observations prove that DSA also operates on scales much larger than in
supernova remnants and that shocks in galaxy clusters are capable of producing
extremely energetic cosmic rays.
