Energy evolution of cosmic-ray mass and intensity measured by the Pierre Auger Observatory

¹ Institute of Particle and Nuclear Physics, Faculty of Mathematics and Physics, Charles University, V Holešovičkách 2, 180 00 Prague 8, Czech Republic
² Observatorio Pierre Auger, Av. San Martín Norte 304, 5613 Malargüe, Argentina

^* e-mail: spokespersons@auger.org

Published online: 6 March 2025

Abstract

The Pierre Auger Observatory has conducted measurements of the energy spectrum and mass composition of cosmic rays using different methods. Utilizing both surface and fluorescence detectors (SD and FD), the Observatory provides unprecedented precision in understanding these particles. While primarily designed to measure ultra-high energy cosmic rays, the FD’s highelevation telescopes and the dense arrays of SD stations enable observations down to 6 PeV and 60 PeV, respectively. To determine the depth of shower maximum, a critical parameter for identifying primary particle types, both direct longitudinal profile measurements from the FD and indirect signal analyses from the SD are employed. An energy evolution of the mass of primary particles, as well as of the spectral index of the flux intensity, are observed and characterized by features described in the presented work. The measurements benefit from the joint operation of the FD and SD, delivering a systematic uncertainty of 14% in energy determination and an accumulated exposure reaching 80 000 km² sr yr at the highest energies.