The Southern Hemisphere site of the Pierre Auger Observatory is now under construction in Argentina by a collaboration of 50 institutions in 16 countries. The objective of the Auger Project is to make a high statistics measurement of cosmic rays above 1019 eV. The observatory will record extensive air showers induced by these cosmic rays incident on the atmosphere. The measurement will include energy, direction and composition of the primary particles. The engineering phase is now complete and full construction has begun.
The search for the source of the highest energy cosmic rays is one of the most interesting problems in astrophysics. Following the discovery of the cosmic microwave background, Greisen and, independently, Zatsepin and Kuzmin realized hat this background radiation would make space opaque to cosmic rays of very high energy. Nevertheless over the past 30 years several tens of events were recorded with energies above the Greisen, Zatsepin, Kuzmin (GZK) cutoff (about 5×1019 eV) including a number above 1020 eV. These events present a conundrum. Because of the GZK cut off these super high-energy events must come from nearby, less than about 50 Mpc. In addition the cosmic acceleration mechanism for achieving these energies is very difficult to conceive. Yet, even though particles of these energies are only slightly deflected by galactic and extragalactic magnetic fields, none clearly points back to a source sufficiently violent to a be a candidate source.
The Auger Observatory finished its engineering development phase at the end of 2001. The “Engineering Array” consists of 40 surface particle detector stations and two prototype air fluorescence telescopes. The Observatory, when complete, will have a 1600 detector surface array covering 3000 km**2 overlooked by 24 fluorescence telescopes. The Engineering Array has demonstrated that all of the detector systems perform as well or better than expected. Recently the Observatory has recorded a number of cosmic ray events simultaneously with both the fluorescence and surface detectors.