When an ultra high-energy cosmic ray enters the upper atmosphere it splinters into an elaborate shower of billions of secondary particles that head for Earth’s surface. While ordinary cosmic rays are a relatively common phenomenon in physics, among astrophysicists these high energy cosmic rays are known as the most elusive of particles due to their rarity and mysterious origins.
They strike so infrequently that one will hit in any given square mile only about once a century. In order to capture them, the Pierre Auger Observatory distributed 1600 5′ x 12′ plastic water tanks across a 3000 square kilometer section of the Argentine Andes. When the particle crosses the tank the difference between the speed of light in air and the speed of light in water creates a shock similar to a supersonic jet, except with light. The result is a flash of light in the water called a Cherenkov emission that their detectors picks up.
While it’s believed that the origin of high energy cosmic rays could be things like supermassive black holes at the center of galaxies, other suspected exotic sources might be dark matter or even extra dimensions. But earlier this year the physicists at the Pierre Auger Observatory uncovered what they now say is a source much closer to home.
The detectors had been finding that many of these ultra high energy particles are nuclei and not protons as had been previously thought. Nuclei are much more fragile than protons and have a tendency to disintegrate and become protons as they travel through space, so the discovery was quite unexpected. But in a paper due to be published in Physical Review Letters, they describe the surprising reason for the disparity: cosmic accelerators in our own galaxy.
The resolution came when the group realized that while stars exploding in the Milky Way can accelerate nuclei and protons alike, the protons would be launched from our galaxy immediately and the much heavier nuclei would get stuck in its magnetic field. As a result there are more nuclei in our vicinity than protons and they are detected in much greater numbers by the Pierre Auger Observatory.
While people have observed the sorts of stellar explosions that would create these high-energy particles in other galaxies, the group says that their discovery shows these explosions happen in our galaxy at least a few times every million years. They also say that the high energy nuclei they see now have been knocking around the galaxy for perhaps millions of years, trapped in a web of galactic magnetic fields.
While the research doesn’t rule out all the other exotic sources for high energy cosmic rays, it does explain the overabundance of nuclei cosmic rays detected on earth.
Here’s a great video by the University of Chicago on the Observatory and its research