Ultra-High Energy Cosmic Rays: Exploring th Universe at the Highest Energies
by Karl-Heinz Kampert (Wuppertal Univ.)
at DESY ( Auditorium )
The origin of Ultra-High Energy Cosmic Rays (UHECR) with energies to beyond 1020 eV is one of the most pressing puzzles in high energy astrophysics. The advent of large scale observatories, most prominently the Pierre Auger Observatory, have dramatically advanced our understanding of UHECR and have brought a number of surprises during the last few years. As an example, a strong suppression of the flux around 5x1019 eV is observed but appears to be caused primarily by the limiting energy of the sources, rather than by the GZK-effect. In consequence, the flux of cosmogenic photons and neutrinos is reduced by some orders of magnitude compared to previous estimates which is a decisive ingredient for estimating the physics potential of future neutrino and gamma-ray observatories. Despite the fact that the UHECR sky turned out to be much more isotropic than expected, significant large-scale anisotropies have recently been observed by the Pierre Auger Observatory at E > 8 EeV which determine the extragalactic origin of these particles. At the highes energies, the sky maps start to exhibit distinct and more complex features. While no discrete sources of UHECR have been identified so far, the noose is tightening around nearby extragalactic objects. Prospects that emerge from this new picture of UHECR will be discussed.