The end of the cosmic ray spectrum

TL;DR

This paper reports on measurements of ultra-high-energy cosmic rays, revealing a spectrum consistent with theoretical predictions, a composition shift from light to heavy nuclei, and anisotropy correlating with nearby extragalactic sources.

Contribution

It provides new observational data on the cosmic-ray spectrum, composition, and anisotropy at energies above 10^18 eV, supporting existing models and suggesting a transition in primary particle types.

Findings

Spectrum shows a flattening above 4x10^18 eV and softening near 3x10^19 eV.

Evidence of a composition shift from light to heavier nuclei with increasing energy.

Detected anisotropy correlating cosmic-ray arrival directions with nearby extragalactic objects.

Abstract

Recent results from the Pierre Auger Observatory are presented, focusing on a measurement of the cosmic-ray energy spectrum above 10^18 eV, cosmic-ray composition, and the anisotropy in the cosmic ray arrival directions. The flux of cosmic rays can be well described by a broken power-law, with a flattening of the spectrum above 4x10^18 eV and a softening of the spectrum beginning at about 3x10^19 eV. The flux suppression at highest energies is consistent with the predictions of Greisen, and Zatsepin and Kuzmin. Longitudinal development of cosmic-ray air showers provides information on the mass of the primary particle. When compared to model predictions, our measurements of the mean and spread of the longitudinal position of the shower maximum are indicating a composition transition from light to heavier with increasing energy. For highest energies in our data-set we observe evidence for a correlation between the cosmic-ray arrival directions and the nearby extragalactic objects.