Measurement of the cosmic ray spectrum above $4{\times}10^{18}$ eV using inclined events detected with the Pierre Auger Observatory

TL;DR

This study measures the ultra-high-energy cosmic ray spectrum above 4×10^{18} eV using inclined showers detected over a decade, confirming flux suppression at the highest energies and characterizing the spectral features.

Contribution

It provides the first detailed spectrum measurement of inclined cosmic-ray showers above 4×10^{18} eV, confirming flux suppression and precisely determining the energy at half flux suppression.

Findings

Flux suppression at the highest energies confirmed.

Spectral index above the ankle measured as 2.70.

Energy at half flux suppression estimated as 5.12×10^{19} eV.

Abstract

A measurement of the cosmic-ray spectrum for energies exceeding $4{\times}10^{18}$ eV is presented, which is based on the analysis of showers with zenith angles greater than $60^{\circ}$ detected with the Pierre Auger Observatory between 1 January 2004 and 31 December 2013. The measured spectrum confirms a flux suppression at the highest energies. Above $5.3{\times}10^{18}$ eV, the "ankle", the flux can be described by a power law $E^{-\gamma}$ with index $\gamma=2.70 \pm 0.02 \,\text{(stat)} \pm 0.1\,\text{(sys)}$ followed by a smooth suppression region. For the energy ($E_\text{s}$) at which the spectral flux has fallen to one-half of its extrapolated value in the absence of suppression, we find $E_\text{s}=(5.12\pm0.25\,\text{(stat)}^{+1.0}_{-1.2}\,\text{(sys)}){\times}10^{19}$ eV.