The supernova remnant SN 1006 as a Galactic particle accelerator

The origin of cosmic rays is a pivotal open issue of high-energy astrophysics. Supernova remnants are strong candidates to be the Galactic factory of cosmic rays, their blast waves being powerful particle accelerators. However, super nova remnants can power the observed flux of cosmic rays only if they transfer a significant fraction of their kinetic energy to the accelerated particles, but conclusive evidence for such efficient acceleration is still lacking. In this sce nario, the shock energy channeled to cosmic rays should induce a higher post shock density than that predicted by standard shock conditions. Here we show this effect, and probe its dependence on the orientation of the ambient magnetic field, by analyzing deep X-ray observations of the Galactic remnant of SN 1006. By comparing our results with state-of-the-art models, we con clude that SN 1006 is an efficient source of cosmic rays and obtain an obser vational support for the quasi-parallel acceleration mechanism.