The origin of ultra-high-energy cosmic rays

Every so often Earth’s atmosphere gets hit by a charged particle (typically an atomic nucleus) with an energy greater than 1 EeV (1018 eV) — in other words, an ultra-high-energy cosmic ray. One of the longest-standing problems in astronomy is the origin of these ultra-high-energy cosmic rays.

°The reason it’s difficult to pinpoint where these particles come from is that the Milky Way’s tangle of magnetic fields sends electrically charged particles in wild spirals. Any directional information about the origin of these charged particles get destroyed. Cosmic rays thus hit Earth equally from all directions, and that begs the question: do these high-energy particles originate within our galaxy or do they have an extragalactic origin? In a recent paper, astrophysicists at the Pierre Auger Laboratory claim to have solved the mystery: ultra-high-energy cosmic rays have an extragalactic origin.

Surface detector

One of the many water-Cherenkov surface detectors that make up the Auger cosmic ray telescope.

The Auger Collaboration looked at 30000 of the very highest-energy particles. Because of their high energies, these particles undergo less deflection than the billions of low-energy cosmic rays that constantly bombard Earth. The Collaboration tracked these particles back, and found that there was an excess of particles coming from a patch of sky 120° away from the centre of the Milky Way. Furthermore, this patch of sky contains a high density of nearby galaxies.

The case seems to be settled: ultra-high-energy cosmic rays have an extragalactic origin. They come from nearby galaxies. Furthermore, since they don’t come from the Milky Way we can probably assume that the galaxies they do come from are somehow different to the Milky Way. Perhaps they originate in a galaxy such as Centaurus A, which contains relativistic jets powered by a supermassive central black hole. Further study will surely pinpoint the precise origin of these enigmatic particles.