The origin of Galactic cosmic rays, their energy source and their acceleration process raise many questions more than 100 years after their discovery by Victor Hess in 1912. What are their sources of acceleration and energy? What are the acceleration mechanisms and their properties?
If these are not the only sources considered, strong shocks in supernova remnants constitute one of the privileged places of acceleration which allows the acceleration of particles by the mechanism of diffusive acceleration. Moreover, if a fraction of 10-20% of the shock kinetic energy is tapped to accelerate the particles, the supernova rate in our Galaxy can account for the energy required to maintain the Galactic cosmic ray population.
X-ray observations, on the other hand, have the advantage of being able to finely map the acceleration locations. And, of major interest, they provide information on the properties of both the thermal plasma heated to millions of degrees and the non-thermal plasma of electrons accelerated to very high energies of the order of teraelectron volts. They thus offer keys to the understanding of shock acceleration mechanisms, and specifically of their feedback and their dependence on the magnetic field.
Understanding the star formation process is a major open question in contemporary astrophysics. It is indeed the process that controls the evolution of galaxies since their birth, gradually transforming their interstellar gas into stars and enriching it with heavy elements and dust grains. It is also the formation of stars that is at the origin of the formation of planetary systems and the appearance of life. This process is however complex and still very poorly understood. It involves the understanding of a series of hydrodynamic instabilities leading to the collapse of a molecular cloud, in which gravity, magnetic field and chemistry play a central role. Moreover, the feedback, i.e. the ionizing radiation and the wind from newly formed massive stars, has the effect of destroying the rest of the molecular cloud and thus inhibiting star formation after a few million years. This feedback is a key element, but it is still insufficiently understood.