Supernovae and binarity 

These galactic cosmic rays are accelerated in massive star environments, mainly in supernova remnants. When the explosion takes place, the core of the star contracts to the point where it becomes a neutron star or a black hole, but the outer layers are violently expelled at very high speeds of up to several thousand kilometers per second. This creates shocks with the surrounding interstellar matter which are capable of accelerating particles. Part of the mechanical energy is transferred to particles (emanating from the star or the surrounding matter) which then reach very high speeds to become cosmic rays. This mechanism is without doubt the principal source of cosmic rays in galaxies.

Michaël De Becker is exploring another avenue of research providing an explanation for particle acceleration, in particular, in the case of low-energy cosmic rays. Their source is massive stars but, more precisely, pairs of massive stars or binary systems. Today we know that a high number of massive stars are made up of double or sometimes even triple systems. The scenario envisaged by the researcher from Liege is therefore not an isolated case but concerns a large number of systems. 
“Binarity is very important because if there are two stars, the stellar winds enter into collision and this gives rise to hydrodynamic shocks which are similar to the shocks generated by supernovae but with a different geometry.  Instead of having a spherical geometry, as in the case of supernova remnants where the shell extends out in all directions,  one is dealing with two shock fronts facing each other and it is in these fronts that particle acceleration occurs”, continues Michaël De Becker.

The capacity of these objects to accelerate particles was revealed by radio observations. In the 1980s, observations revealed anomalies in the radio emissions of certain massive stars. These anomalies revealed synchrotron radiation, that is to say, emitted by electrons which are accelerated to a speed close to that of light in the presence of a magnetic field. This confirmed both the presence of a magnetic field but also that of relativistic electrons and therefore the presence of a powerful acceleration mechanism as the electrons did not have this energy previous to this.

Michaël De Becker continues, “We can therefore say that massive stars are involved in the formation of low-energy galactic cosmic rays on two levels. Firstly, during the several millions years-often around ten- which precede the supernova explosion, these massive binary systems are capable of acting as particle accelerators and then again in the supernova remnant phase. Certainly, during the millions of years of life of binary systems, particle acceleration is less efficient than in the case of supernova remnants, but the remnant is only efficient for a few thousand years”.