An article published in the magazine “Science” describes a research on the distribution of cosmic rays’ arrival directions. The Pierre Auger Collaboration used data collected by the Pierre Auger Observatory, Argentina, the largest ever built to detect cosmic rays, to find evidence that high energy cosmic rays come from outside the Milky Way.

Cosmic rays, made up for about 90% by protons and for the rest by electrons and nuclei of various atoms, have been a mystery for decades and among the questions many researchers are seeking to answer there’s their origin. the Pierre Auger Collaboration, with over 400 scientists from 18 different nations, focused on cosmic rays with an energy greater than 10^20 electronvolts, which means over 10 million times greater than that of the protons that are accelerated within the Large Hadron Collider.

Such high-energy particles are rare: in simple terms one of them strikes the surface of a football field once every 120 years. They’re detectable because they interact with the nuclei of the elements in the Earth’s atmosphere, generating electrons, photons and muons and then other particles that emit Cherenkov light passing through some of the 1.600 detectors of the Pierre Auger Observatory that contain 12 tons of water each.

The total area covered by the detectors is 3,000 square kilometers. Additionally, four telescope stations that control the ground detectors are added. The particles’ arrival times to the detectors are measured with GPS receivers and used to determine the direction from which the particles arrived at one degree approximation.

That was crucial for Pierre Auger Collaboration’s researchers to discover an anisotropy with extreme precision. Anisotropy is a technical term that indicates a inhomogeneity in the arrival directions of over 30,000 of those particles, with differences in the amount of cosmic rays coming from different directions.

Thanks to that precision, it was possible to determine that the probability that the detected anisotropy is accidental is about two in ten million. An abnormal amount of high energy cosmic rays comes from one direction, circled in the image, where the distribution of galaxies is relatively high. However, a precise identification of their origin is impossible because the particles are deflected by the galactic magnetic field.

A further research is already underway, based on cosmic rays with even higher energies. They’re very rare but because of their energy they suffer a lower deflection so their analysis would allow to locate more precisely the area they came from. This in turn would provide information on the violent phenomena that generated such energetic particles helping other types of astrophysical research.