Different models have been proposed to explain giant radio halos. For instance, cosmic ray protons, like primary electrons, can be accelerated in the accretion shocks of active galactic nuclei. Unlike primary electrons, they lose energy less efficiently, building up the cluster radio halo. However, this model cannot explain the complexity of the observed giant radio halo characteristics. A second possibility is that energetic electrons responsible for non-thermal emissions originate from lower-energy electrons that are re-accelerated by turbulence generated as a consequence of cluster mergers. Astronomers working on the GIANT RADIO HALOS project calculated for the first time the re-acceleration of cosmic ray electrons and their synchrotron emission from first principles. For this purpose, they defined a model for compressible magnetohydrodynamic (MHD) turbulence in the intra-cluster medium. The non-linear evolution of the cosmic ray spectrum was reproduced by a Fokker-Planck equation, describing statistical properties of particle motion. To compute the evolution of a cosmic ray spectrum resulting from turbulence, they considered isotropic particle pitch-angle distributions and took into account both particle energy gains and losses. MHD simulations of a single cluster merger allowed astronomers to follow the evolution of relativistic electron spectra and radio emission generated. The simulated re-acceleration was sufficient to boost the total observable radio emission of the centre of the system by a factor of more than 100. The resulting morphology and timeline of the radio emission was consistent with that of giant radio halo observations. Although the simulation was based on a very simple assumption, it has paved the way to a more detailed examination of turbulent acceleration of relativistic electrons with more advanced numerical simulations.
Giant radio halos, non-thermal emissions, galaxy clusters, cosmic ray, magnetohydrodynamic