  Galaxy clusters are the most massive gravitationally bound structures in the
Universe. They grow by accreting smaller structures in a merging process that
produces shocks and turbulence in the intra-cluster gas. We observed a ridge of
radio emission connecting the merging galaxy clusters Abell 0399 and Abell 0401
with the Low Frequency Array (LOFAR) at 140 MHz. This emission requires a
population of relativistic electrons and a magnetic field located in a filament
between the two galaxy clusters. We performed simulations to show that a
volume-filling distribution of weak shocks may re-accelerate a pre-existing
population of relativistic particles, producing emission at radio wavelengths
that illuminates the magnetic ridge.
