A Large-Area Survey of Ultrahigh-Energy Cosmic Rays
Cosmic rays strike Earth from all angles and with energies that span many orders of magnitude. The precise distribution and spectrum of these particles—protons and heavier nuclei—offer clues to the particles’ makeup and origins. In the past decade, surveys of ultrahigh-energy cosmic rays conducted by the Pierre Auger Observatory in Argentina revealed a new spectral feature known as the instep. Located between the previously identified “ankle” and “toe,” the instep hinted that most of the cosmic-ray flux above about 10 exa-electron-volts originates in nuclei heavier than hydrogen (see Viewpoint: The Anatomy of Ultrahigh-Energy Cosmic Rays). The Pierre Auger Observatory Collaboration has now augmented those results with a campaign that takes in a larger area of the sky [1].
As before, the new data were obtained using the facility’s array of water-filled tanks spread over an area of about 3000 km2. Each of the tanks is lined with sensors that pick up Cherenkov radiation from the fast-moving muons created when a cosmic-ray particle strikes the upper atmosphere. Reconstructing the properties of an incoming cosmic ray from these detections entails accounting for Earth’s magnetic field. Because this adjustment becomes harder as the zenith angle approaches 90° (arrival just above the horizon), previous measurements included only events with zenith angles less than 60º. The new data include events up to 80º, meaning approximately 75% of the entire sky has now been observed. The instep showed up in every direction. This spatial uniformity suggests that the feature results from cosmic-ray accelerators that are numerous, extragalactic, and governed by similar physical mechanisms.
–Marric Stephens
Marric Stephens is a Corresponding Editor for Physics Magazine based in Bristol, UK.
References
- A. Abdul Halim et al. (Pierre Auger Collaboration), “Energy spectrum of ultrahigh-energy cosmic rays across declinations –90º to +44.8º as measured at the Pierre Auger Observatory,” Phys. Rev. Lett. 135, 241002 (2025).