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

TL;DR

This paper reports on the Pierre Auger Observatory's measurements of cosmic-ray energy spectrum and composition, revealing how the mass and intensity evolve with energy using combined surface and fluorescence detection methods.

Contribution

It presents new high-precision measurements of cosmic-ray energy spectrum and mass composition across a wide energy range, utilizing combined detection techniques.

Findings

Observed energy evolution of primary particle mass.

Characterized spectral index variations with energy.

Achieved systematic energy uncertainty of 14%.

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 high-elevation 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 km$^2$ sr yr at the highest energies.