Each second, about 200 cosmic ray particles with energies of a few million electron volts strike every square meter of the Earth. While these low-energy cosmic rays are plentiful, those with higher energies are far rarer. Events have been observed with an energy 100 million times higher than those produced by the most powerful particle accelerator on Earth, but at a rate of only a few per square kilometer per millenium! Instruments with extremely large collecting areas are deployed to sample the shower of secondary particles produced when the primary cosmic ray collides with the Earth atmosphere. The Pierre Auger Observatory stretches over 3000 square kilometers in western Argentina, roughly the size of Rhode Island. It measures extensive air showers both on the ground with 1600 detectors space 1.5 km apart and in the air, viewing the brief flash of nitrogen molecules de-exciting after the shower passes by.
Scientists of the Pierre Auger Collaboration announced today (8 Nov. 2007) that active galactic nuclei are the most likely candidate for the source of the highest-energy cosmic rays that hit Earth. The team of scientists from 17 countries found that the sources of the highest-energy particles are not distributed uniformly across the sky. Instead, their results link the origins of these mysterious particles to the locations of nearby galaxies that have active nuclei in their centers. The results will appear in the Nov. 9 issue of the journal Science.
Active Galactic Nuclei (AGN) are thought to be powered by supermassive black holes that are devouring large amounts of matter. They have long been considered sites where high-energy particle production might take place. They swallow gas, dust and other matter from their host galaxies and spew out particles and energy. While most galaxies have black holes at their center, only a fraction of all galaxies have an AGN. The exact mechanism of how AGNs can accelerate particles to energies 100 million times higher than the most powerful particle accelerator on Earth is still a mystery.