The Pierre Auger Observatory

Construction of the southern site of the Pierre Auger Observatory is starting now in Argentina. It is expected that in two years the construction of the northern site will begin. The two sites together will comprise an area of 6000 km(2) covered with 3200 water Cherenkov detectors and 8 fluorescence detectors with an aperture and a duty cycle large enough to collect in few years several hundreds of cosmic rays with energies around and beyond the GZK cut-off, that is with energies greater than or equal to 5 x 10(19) eV. This sample will allow the Pierre Auger Collaboration to establish with confidence the spectrum and the direction of the incoming cosmic rays as well as to identify statistically the type of particles that compose the flux of cosmic rays in this part of the spectrum.