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

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 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.

Ultra-High-Energy Cosmic Rays: The Intersection of the Cosmic and Energy Frontiers

“…Each FD telescope consists of a 13 m 2 -area curved mirror focusing the fluorescence light emitted in air showers onto a camera composed of 440 hexagonal PMTs, and has a 30 • × 30 • field of view (FoV) with a minimum elevation of 1.5 • above the horizon. In order to extend the sensitivity to lowerenergy showers, in a 23.5 km 2 region of the array, 61 SD stations have been deployed with a 750 m spacing ("SD-750") [48] and 19 stations with a 433 m spacing ("SD-433") [49], overlooked by three extra FD telescopes looking at elevations of 30 • to 58 • above the horizon (High Elevation Auger Telescope (HEAT)). The Observatory also contains various other facilities for calibration, atmosphere monitoring, R&D, and interdisciplinary purposes, such as the Auger Engineering Radio Array (AERA).…”

“…The low-energy extensions of the Observatory allows studies to be extended into the energy range where Galactic cosmic rays are expected to dominate. The SD-750 has a detection efficiency of approximately 100% for events with θ < 40 • and E ≥ 10 17 eV, and has been used to measure the energy spectrum of cosmic rays down to the so-called second-knee [48]. The SD-433 will extend the full efficiency further to 10 16.6 eV [49], while preliminary studies using the HEAT FD to detect the air Cherenkov emissions from showers reach down to 10 15.8 eV, below the so-called low-energy ankle [66].…”

High-Energy and Ultra-High-Energy Neutrinos

“…Figure 14. The high-energy astrophysical multi-messenger spectrum including extragalactic γ-ray spectrum measured by Fermi-LAT [539], the ultra-high energy cosmic ray spectrum measured by the Pierre Auger Observatory [540] and the Telescope Array [541], and the high-energy neutrino measurements by IceCube [21,274]. The filled and halo blue bands represent the range of neutrino fluxes obtained in [274] and [357] assuming a single power-law diffuse spectrum.…”

Measurement of Energy Spectrum and Elemental Composition of PeV Cosmic Rays: Open Problems and Prospects

“…In Figure 8 recent measurements of the all-particle energy spectrum down to about 100 PeV by the Pierre Auger Observatory are also reported. These observations suggest that the second knee is not a sharp feature, but a softening that extends in the interval 100-200 PeV [66]. Preliminary results based on the distribution of the depth of the shower maximum are consistent with a spectrum dominated by heavy nuclei in the 10 17 eV range becoming lighter with increasing energy [67].…”