A giant radio halo in the cool core cluster CL1821+643
  Giant radio halos are Mpc-size sources found in some merging galaxy clusters.
The synchrotron emitting electrons are thought to be (re)accelerated by plasma
turbulence induced by the merging of two massive clusters. Cool core galaxy
clusters have a low temperature core, likely an indication that a major merger
has not recently occurred. CL1821+643 is one of the strongest cool core
clusters known so far. Surprisingly, we detect a giant radio halo with a
largest linear size of $\sim$ 1.1 Mpc. We discuss the radio and X-ray
properties of the cluster in the framework of the proposed models for giant
radio halos. If a merger is causing the radio emission, despite the presence of
a cool-core, we suggest that it should be off-axis, or in an early phase, or a
minor one.
