On the origin of the ultra-high energy cosmic-rays

Cosmic-rays are one of the most fascinating phenomena in the universe. They consist of energetic particles with an out-of-equilibrium power-law spectrum extending over at least eleven orders of magnitude in energy, from ~1 GeV to 10^11 GeV. In the past decade, new measurements by experiments such as the Pierre Auger observatory and Telescope Array, have greatly improved our knowledge of the highest energy domain of the cosmic-ray spectrum, the "ultra-high energy cosmic-rays" (UHECR), with energies > 10^9 GeV. At these energies, cosmic-rays are thought to be of extragalactic origin and they are highly challenging by questions with respect to their origins and their acceleration processes. 

I will first review the observational data on the cosmic-ray spectrum, composition and arrival directions. I will show that the spectrum and composition can be explained by a generic model having one Galactic component and one extragalactic component. I will review the multi-messenger constrains brought by neutrino and gamma-ray experiments on UHECR origin. Finally, I will discuss the origin of the UHECR dipole anisotropy recently reported by the Pierre Auger Observatory.