The Pierre Auger Observatory observes surprising behavior in the most energetic cosmic rays
The Auger collaboration, which operates a 3,000 km² cosmic ray observatory in the Argentine Andes, has announced the observation of an unexpected structure in the spectrum of ultra-high-energy cosmic rays, above 10¹⁸ electron volts. This break, known as the “instep,” suggests that the frequency of cosmic rays suddenly decreases more sharply beyond 10 exaelectronvolts (EeV). In addition, the collaboration observes that the inflections in the spectrum are identical in all directions, which would imply that they are emitted by the same extragalactic sources throughout the Universe.
For more than 20 years, the Auger collaboration, in which several CNRS Nuclei & Particles groups participate, has been investigating cosmic rays, particles from space that constantly bombard our atmosphere. For more than 20 years, the network of Cherenkov detectors covering 3,000 km² has been capturing ultra-high-energy cosmic rays (UHECRs) with the aim of one day understanding where these cosmic messengers come from and which celestial behemoths send them to us. The study published on December 9 in the journal Physical Review Letters by the collaboration, without yet solving the mystery of the source of UHECRs, is an important step toward understanding this phenomenon by more precisely characterizing their frequency and distribution.
UHECRs are made up of particles of matter, such as protons or atomic nuclei accelerated to energies that would make CERN's LHC salivate. Their wild ride through the cosmos ends when they produce a shower of particles upon contact with the Earth's atmosphere. When one of these showers unfolds in the sky over the Argentine Andes, a portion of these particles—muons, electrons, and photons—will be captured by hundreds of Cherenkov detection tanks and, for the nocturnal ones, by the four fluorescence telescopes of the Auger collaboration. By analyzing these showers, physicists will be able to glean crucial information, such as the energy and direction of UHECRs. The results published in Physical Review Letters are based on a compilation of 20 years of data patiently collected by the collaboration.
It confirms the existence of a phenomenon known as the “instep” in the UHECR spectrum, i.e., the curve that describes the frequency of events as a function of their energy. Previous studies had already hinted at the existence of this break between two long-known breaks—the ankle and the toe—and its existence in the UHECR spectrum is now beyond doubt. This “instep” reflects a marked decrease in the frequency of UHECRs above 10 exaelectronvolts, the energy level at which the majority of cosmic rays would consist of nuclei heavier than hydrogen.
But the Auger team's new analysis does more than just confirm the existence of this structure. Until now, only particles arriving at a zenith angle of less than 60° were considered, because signal reconstruction becomes more complex when the arrival angle is low, due to the effects of the Earth's magnetic field, which modifies the trajectory of charged particles. This time, thanks to the development of layers that allow events to be reconstructed taking into account the influence of the magnetic field, the team extended its study to 80°, covering approximately 75% of the entire sky. This massive increase in the observed area provides a much more complete and statistically robust panorama.
However, the result is clear: the instep discovered by the collaboration appears everywhere, regardless of the region of the sky observed. This uniformity strongly suggests that this structure does not originate from an isolated source or a local phenomenon, but rather from a set of numerous extragalactic cosmic accelerators, operating according to similar physical processes. In other words, the most energetic particles reaching our atmosphere appear to be produced in various objects located far beyond the Milky Way, probably in extreme astrophysical environments.
The homogeneity identified by Auger represents a valuable new constraint on theoretical models that seek to explain how these particles can be accelerated to such energies. Theoretical efforts will soon benefit from the flood of data from an observatory with improved performance thanks to the AugerPrime upgrade, which will be operational very soon.