Catching the highest energy cosmic rays

In the 2008 George Darwin Lecture of the Royal Astronomical Society, Alan Watson describes how the Pierre Auger Observatory detects and decodes the very highest energy cosmic rays and what this tells us about their mysterious origins.The Pierre Auger Observatory in the Argentinian pampas is a vast hybrid instrument combining fluorescence telescopes and surface water-Cherenkov detectors, the product of a major international collaboration. This article describes the science that underpins the design and operation of the observatory, now fully functioning nearly 10 years after the start of construction. Early data from the complete array suggest an association between the sources of very high energy cosmic rays and active galactic nuclei.