The energy spectrum of cosmic rays beyond the turn-down around $10^{17}$ eV as measured with the surface detector of the Pierre Auger Observatory

TL;DR

This paper reports a precise measurement of the cosmic-ray energy spectrum above 100 PeV, confirming the second-knee feature and linking it with other spectral breaks at higher energies, advancing understanding of ultra-high-energy cosmic rays.

Contribution

It provides the most precise measurement of spectral breaks in the cosmic-ray spectrum above 100 PeV, using combined data from surface detectors and fluorescence telescopes.

Findings

Confirmation of the second-knee feature in the spectrum.

Identification of multiple spectral breaks at the highest energies.

Most statistically and systematically precise measurement to date.

Abstract

We present a measurement of the cosmic-ray spectrum above 100\,PeV using the part of the surface detector of the Pierre Auger Observatory that has a spacing of 750~m. An inflection of the spectrum is observed, confirming the presence of the so-called \emph{second-knee} feature. The spectrum is then combined with that of the 1500\,m array to produce a single measurement of the flux, linking this spectral feature with the three additional breaks at the highest energies. The combined spectrum, with an energy scale set calorimetrically via fluorescence telescopes and using a single detector type, results in the most statistically and systematically precise measurement of spectral breaks yet obtained. These measurements are critical for furthering our understanding of the highest energy cosmic rays.