Ultra-high energy cosmic rays

AbstractExperimental efforts to reveal the nature and origin of cosmic rays with energy above 1018 eV led to several important steps forward in recent years. The existence of a suppression of the flux above 4×1019 eV has been confirmed. It occurs at the energy threshold for pion-production in proton collisions with the cosmic microwave background, as anticipated almost fifty years ago. The flux measurements alone are however insufficient to confidently establish whether the suppression is due to energy loss effects along propagation over cosmological distances, or else because the sources reach their maximum acceleration power. There are indications obtained with the Pierre Auger Observatory of a trend from a light towards a heavier composition as the energy increases. There is some tension between these indications and those from the HiRes and Telescope Array experiments, which are compatible with a pure proton composition. This is a most important issue to be settled in the near future. At present there is no statistically significant evidence for anisotropy in the distribution of arrival directions at the highest energies that could favor one specific astrophysical scenario for cosmic ray origin over another. There are hints for a large scale pattern in the distribution of arrival directions that need to be confirmed with independent data. In this paper we summarize recent measurements of the energy spectrum of cosmic rays with the highest energies, the evidence for their composition, and the searches for anisotropies in the distribution of their arrival directions