The Highest- Energy Cosmic Rays.

Probably come from the cores of active galactic nuclei (AGN), where supermassive black holes are thought to supply vast energy for flinging the rays across the cosmos. This is the conclusion reached by scientists who operate the Pierre Auger Observatory in Argentina. This gigantic array of detectors spread across 3000 sq. km of terrain, looks for one thing: cosmic ray showers.

These arise when extremely energetic particles strike our atmosphere, spawning a gush of secondary particles. Many of the rays come from inside our own Milky Way, especially from our sun, but many others come from far away. Of most interest are the highest-energy showers, with energies above 10^19 electron volts, far higher than any particle energy that can be produced in terrestrial accelerators. The origin of such potent physical artifacts offers physicists a tool for studying the most violent events in the universe.

To arrive at Earth most cosmic rays will have crossed a great deal of intergalactic space, where magnetic fields can deflect them from their starting trajectories. But for the highest-energy rays, the magnetic fields can’t exert as much influence, and consequently the starting point for the cosmic rays can be traced with some confidence.

This allowed the Auger scientists to assert that the premier cosmic rays were not coming uniformly from all directions but rather preferentially from galaxies with active cores, where the engine for particle acceleration was probably black holes of enormous size. The very largest of cosmic ray showers, those with an energy higher than 57 EeV (1EeV equals 10^18 eV), correlated pretty well with known AGN’s. (Science, 9 November 2007)