Discovery of radio halos and double-relics in distant MACS galaxy
  clusters: clues to the efficiency of particle acceleration
  We have performed 323 MHz observations with the Giant Metrewave Radio
Telescope of the most promising candidates selected from the MACS catalog. The
aim of the work is to extend our knowledge of the radio halo and relic
populations to z>0.3, the epoch in which massive clusters formed. In
MACSJ1149.5+2223 and MACSJ1752.1+4440, we discovered two double-relic systems
with a radio halo, and in MACSJ0553.4-3342 we found a radio halo. Archival Very
Large Array observations and Westerbork Synthesis Radio Telescope observations
have been used to study the polarization and spectral index properties. The
radio halo in MACSJ1149.5+2223 has the steepest spectrum ever found so far in
these objects (alpha > 2). The double relics in MACSJ1149.5+2223 are peculiar
in their position that is misaligned with the main merger axis. The relics are
polarized up to 30% and 40% in MACSJ1149.5+2223 and MACSJ1752.040+44,
respectively. In both cases, the magnetic field is roughly aligned with the
relics' main axes. The spectra in the relics in MACSJ1752.040+44 steepen
towards the cluster centre, in agreement with model expectations. X-ray data on
MACSJ0553.4-3342 suggests that this cluster is undergoing a major merger, with
the merger axis close to the plane of the sky. The cores of the disrupted
clusters have just passed each other, but no radio relic is detected in this
system. If turbulence is responsible for the radio emission, we argue that it
must develop before the core passage. A comparison of double relic plus halo
system with cosmological simulations allows a simultaneous estimate of the
acceleration efficiencies at shocks (to produce relics) and of turbulence (to
produce the halo).
