The Pierre Auger Observatory - a new stage in the study of the ultra-high energy cosmic rays

Abstract

This paper presents the highlights from the recent measurements of the energy spectrum, mass composition, and arrival directions of the ultra-high energy ($> 10^{17}$ eV = 0.1 EeV) cosmic rays (UHECR) by the Pierre Auger Observatory. The Auger hybrid detector combines fluorescence observations of the longitudinal profiles of extended air showers, initiated by these extremely energetic particles, with measures of the shower secondaries at the ground level by its large array of Cherenkov water tanks. The complementariness of the two techniques provides important cross-checks at the energies unreachable with accelerator experiments. The Pierre Auger experiment has observed a strong steepening of the cosmic ray flux above 40 EeV. A significant anisotropy in the arrival directions of cosmic rays has been established for events above $\simeq 60$ EeV. These events correlate over angular scales of less than 6$^\circ$ with the directions towards nearby ($D < 100$ Mpc) active galactic nuclei. Both observations are consistent with the standard scenarios of the UHECR production in the extragalactic astrophysical acceleration sites. The derived stringent upper limits on the photon and neutrino contents in the UHECR are additional important arguments in support of these scenarios. The measurements of the variation of the depth of shower maximum with energy, interpreted with the current hadronic interaction models, favour the mixed cosmic ray composition at energies above 0.4 EeV. We describe the plans and prospects for the future, that will further help to solve the 50 years old UHECR puzzle.