Using more than twenty years of data collected with the Pierre Auger Observatory, the Auger Collaboration has measured the energy spectrum of ultra-high-energy cosmic rays with unprecedented precision. The results, recently published in Physical Review Letters, reveal a previously hinted-at but now clearly established feature in the spectrum at energies around 1019 eV.
The Pierre Auger Observatory, located in Argentina, is the world’s largest detector for cosmic rays. It combines a surface array of water-Cherenkov detectors spread over 3000 km² with fluorescence telescopes that observe the development of extensive air showers in the atmosphere. This hybrid design allows a precise reconstruction of the energy of the primary particles.
The measured spectrum (see Figure 1) shows a marked change in behaviour above 10¹⁹ eV, where the flux of cosmic rays decreases more rapidly with energy. This feature, sometimes referred to as an "instep", appears in addition to the well-known structures of the spectrum observed at lower energies.
Figure 1. Energy spectrum scaled by E2 (energy flux) across declinations [−90°,+44.8°]. The number of events is indicated in each bin. The red band stands for the systematic uncertainties while the dotted line indicates the best-fit function described by the spectral features given with their statistical and systematic uncertainties.
A key aspect of this analysis is the combination of very inclined events (zenith angles between 60° and 80°, see an example in Figure 2, right) with vertical ones (zenith angle between 00° and 60°, see Figure 2, left). This significantly increases the exposure of the observatory and allows the spectrum to be studied over about 75 % of the sky. The feature at 1019 eV is found to be consistent across all observed directions.
Figure 2. Left: Display of the footprint of a “vertical” shower detected with 18 detectors of the surface array and two units of the fluorescence detector. The energy is 72 EeV and the zenith angle is 44 deg. Right: Display of the footprint of an “inclined” shower detected with 75 detectors of the surface array. The energy is 93 EeV and the zenith angle is 79 deg.
The uniformity of this behaviour over the sky suggests that it does not originate from a small number of nearby sources. Instead, it points toward a population of extragalactic sources that accelerate particles to extreme energies through similar physical mechanisms. These new results provide important constraints on models of cosmic-ray acceleration and propagation at the highest energies. Future measurements, in particular those enabled by the AugerPrime upgrade, will enable the energy spectrum of the various elements that make up ultra-high energy cosmic rays to be studied.
Related Paper:
The Energy Spectrum of Ultra-High Energy Cosmic Rays across Declinations −90° to +45°
The Pierre Auger Collaboration, Phys. Rev. Lett. 135, 241002 (2025) "Editor’s suggestion" and Physics Magazine
[arxiv.org/abs/2506.11688]
This paper was also highlighted in the Physics Magazine.
