Cosmic ray transport in galaxy clusters: implications for radio halos
  and gamma-rays
  Observations of giant radio halos provide unambiguous evidence for the
existence of cosmic ray (CR) electrons and magnetic fields in galaxy clusters.
The physical mechanism generating radio halos is still heavily debated. We
critically discuss the proposed models for the radio halo emission and
highlight the weaknesses underlying each explanation. We present an idea how
the interplay of CR propagation and turbulent advection selects a bimodal
spatial CR distribution that is characteristic for the dynamical state of a
cluster. As a result, strongly turbulent, merging clusters should have a more
centrally concentrated CR energy density profile with respect to relaxed ones
with very subsonic turbulence. This translates into a bimodality of the
expected diffuse radio and gamma ray emission of clusters. Thus, the observed
bimodality of cluster radio halos appears to be a natural consequence of the
interplay of CR transport processes, independent of the model of radio halo
formation, be it hadronic interactions of CR protons or re-acceleration of
low-energy CR electrons.
