Abstract
Cluster collisions are a frequent occurrence, giving rise to relict radio emissions that can be spectacular. In addition to the hot equilibrium gas of the Viriel (thermal plasma), there is also a non-thermal plasma arising from the radio jets of the active nuclei. These jets emit synchrotron emission, which is that of relativistic electrons in a magnetic field. Electrons are emitted into the jet with a power-law N(E) energy distribution. The most energetic electrons lose their energy faster, and the spectrum becomes steeper. Similarly, the spectrum of synchrotron emission is a power law that becomes steeper as the jet ages. After a relatively short time (a few tens of millions of years), the jet becomes invisible. When shock waves are produced by collisions between clusters, relativistic electrons are accelerated by the shock, and even several times: an electron trapped in a magnetic mirror can move back and forth and cross the shock thousands of times, giving it back very high energy. This is how we see radio halos and radio relics.
Relics are often located at the edges of clusters, and are sometimes symmetrical on either side. They can be impressive in size, comparable to the diameter of the cluster itself. Along the arc-shaped shock, it is possible to see a gradient in the slope of the power spectrum, showing that the electrons were first accelerated from inside the arc, then the acceleration propagates towards the edge (less steep). The polarization of the radiation shows the direction of the magnetic field, parallel to the arc. Accompanying the synchrotron emission are X-rays, which result from the inverse Compton process: relativistic electrons give energy to low-frequency photons, which become X-rays. The Psynchrotron/Pcompton power ratio then gives a value for the Magnetic Energy/Energy ratio in the photons. The magnetic field at the center of clusters is of the order of 10 micro-gauss. In addition to shock waves, there are also cold fronts, which can provide information on the collision history of the cluster.
