💥 Observation of surprising behavior in the most energetic cosmic rays

The Auger collaboration, which operates a cosmic ray observatory spanning 3000 km² (~1158 sq mi) in the Argentine Andes, announces the observation of an unexpected structure in the spectrum of ultra-high-energy cosmic rays, beyond 10¹⁸ electronvolts.

This break, dubbed the "instep," suggests that the frequency of cosmic rays drops sharply beyond 10 exaelectronvolts (EeV). Furthermore, the collaboration observes that the inflections in the spectrum are found identically in all directions, which would imply that they are emitted by the same extragalactic sources everywhere in the Universe.


For over 20 years, the Auger collaboration, which includes several CNRS Nuclear & Particles groups, has been investigating cosmic rays, these particles from space that constantly bombard our atmosphere. For over 20 years, the network of Cherenkov detectors spread over 3000 km² (~1158 sq mi) has been capturing ultra-high-energy cosmic rays (UHECR), with the aim of one day understanding where these cosmic messengers come from and what celestial behemoths send them to us.

The study published on December 9 in the journal Physical Review Letters by the collaboration, while not yet solving the mystery of the source of UHECR, constitutes an important step towards understanding this phenomenon by more precisely characterizing their frequency and distribution.

UHECR are made of matter particles, such as protons or atomic nuclei accelerated to energies that would make CERN's LHC envious. Their frantic race 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 night sky of the Argentine Andes, a portion of these particles—muons, electrons, and photons—is captured by the hundreds of detection tanks and the four Cherenkov telescopes of the Auger collaboration. From the analysis of these showers, physicists can trace back to crucial information, such as the energy and direction of the UHECR. The result published in Physical Review Letters is based on a compilation of 20 years of this data patiently collected by the collaboration.

It confirms the existence of a phenomenon dubbed the "instep" in the spectrum of UHECR, that is, the curve describing the frequency of events as a function of their energy. Previous studies had already hinted at the existence of this break located 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 UHECR beyond 10 exaelectronvolts, an energy at which the majority of the cosmic ray flux would consist of nuclei heavier than hydrogen.

But the new analysis by the Auger collective does not merely confirm the existence of this structure. Until now, only particles arriving with a zenith angle less than 60° were considered, because signal reconstruction becomes more complex when the arrival is grazing, due to the effects of the Earth's magnetic field, which alters the trajectory of charged particles. This time, thanks to the development of techniques allowing the reconstruction of events while accounting for the influence of the magnetic field, the team extended its study up to 80°, allowing coverage of about 75% of the entire sky. This massive increase in the observed area offers a much more complete and statistically robust panorama.

Now, the result is clear: the instep discovered by the collaboration appears everywhere, regardless of the observed region of the sky. This uniform character 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 that reach our atmosphere seem to be produced in various objects located well beyond the Milky Way, probably in extreme astrophysical environments.

The homogeneity identified by Auger represents a new, valuable constraint for theoretical models that seek to explain how these particles can be accelerated to such energies. Theoretical efforts that will soon benefit from the flow of data from an observatory with improved performance from AugerPrime, which will be operational very soon.