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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.

Abstract The hybrid design of the Pierre Auger Observatory allows for the measurement of the properties of extensive air showers initiated by ultra-high energy cosmic rays with unprecedented precision. By using an array of prototype underground muon detectors, we have performed the first direct measurement, by the Auger Collaboration, of the muon content of air showers between$$2\\times 10^{17}$$2×1017 and$$2\\times 10^{18}$$2×1018 eV. We have studied the energy evolution of the attenuation-corrected muon density, and compared it to predictions from air shower simulations. The observed densities are found to be larger than those predicted by models. We quantify this discrepancy by combining the measurements from the muon detector with those from the Auger fluorescence detector at$$10^{{17.5}}\\, {\\mathrm{eV}} $$1017.5eV and$$10^{{18}}\\, {\\mathrm{eV}} $$1018eV . We find that, for the models to explain the data, an increase in the muon density of$$38\\%$$38%$$\\pm 4\\% (12\\%)$$±4%(12%)$$\\pm {}^{21\\%}_{18\\%}$$±18%21% for EPOS-LHC, and of$$50\\% (53\\%)$$50%(53%)$$\\pm 4\\% (13\\%)$$±4%(13%)$$\\pm {}^{23\\%}_{20\\%}$$±20%23% for QGSJetII-04, is respectively needed.

The hybrid design of the Pierre Auger Observatory allows for the measurement of the properties of extensive air showers initiated by ultra-high energy cosmic rays with unprecedented precision. By using an array of prototype underground muon detectors, we have performed the first direct measurement, by the Auger Collaboration, of the muon content of air showers between$$2\\times 10^{17}$$2 × 10 17 and$$2\\times 10^{18}$$2 × 10 18 eV. We have studied the energy evolution of the attenuation-corrected muon density, and compared it to predictions from air shower simulations. The observed densities are found to be larger than those predicted by models. We quantify this discrepancy by combining the measurements from the muon detector with those from the Auger fluorescence detector at$$10^{{17.5}}\\, {\\mathrm{eV}} $$10 17.5 eV and$$10^{{18}}\\, {\\mathrm{eV}} $$10 18 eV . We find that, for the models to explain the data, an increase in the muon density of$$38\\%$$38 %$$\\pm 4\\% (12\\%)$$± 4 % ( 12 % )$$\\pm {}^{21\\%}_{18\\%}$$± 18 % 21 % for EPOS-LHC , and of$$50\\% (53\\%)$$50 % ( 53 % )$$\\pm 4\\% (13\\%)$$± 4 % ( 13 % )$$\\pm {}^{23\\%}_{20\\%}$$± 20 % 23 % for QGSJetII-04 , is respectively needed.

The hybrid design of the Pierre Auger Observatory allows for the measurement of the properties of extensive air showers initiated by ultra-high energy cosmic rays with unprecedented precision. By using an array of prototype underground muon detectors, we have performed the first direct measurement, by the Auger Collaboration, of the muon content of air showers between 2 × 1017 and 2 × 1018 eV. We have studied the energy evolution of the attenuation-corrected muon density, and compared it to predictions from air shower simulations. The observed densities are found to be larger than those predicted by models. We quantify this discrepancy by combining the measurements from the muon detector with those from the Auger fluorescence detector at 1017.5 eV and 1018 eV. We find that, for the models to explain the data, an increase in the muon density of 38% ±4%(12%) ±^(21%)_(18%) for EPOS-LHC, and of 50%(53%) ±4%(13%) ±^(23%)_(20%) for QGSJETII-04, is respectively needed.

Cosmic rays are high-energy particles arriving from space; some have energies far beyond those that human-made particle accelerators can achieve. The sources of higher-energy cosmic rays remain under debate, although we know that lower-energy cosmic rays come from the solar wind. The Pierre Auger Collaboration reports the observation of thousands of cosmic rays with ultrahigh energies of several exa–electron volts (about a Joule per particle), arriving in a slightly dipolar distribution (see the Perspective by Gallagher and Halzen). The direction of the rays indicates that the particles originated in other galaxies and not from nearby sources within our own Milky Way Galaxy. Science , this issue p. 1266 ; see also p. 1240 The highest-energy cosmic rays have a dipolar distribution, showing that they originate outside our Galaxy. Cosmic rays are atomic nuclei arriving from outer space that reach the highest energies observed in nature. Clues to their origin come from studying the distribution of their arrival directions. Using 3 × 10 4 cosmic rays with energies above 8 × 10 18 electron volts, recorded with the Pierre Auger Observatory from a total exposure of 76,800 km 2 sr year, we determined the existence of anisotropy in arrival directions. The anisotropy, detected at more than a 5.2σ level of significance, can be described by a dipole with an amplitude of 6.5 − 0.9 + 1.3 percent toward right ascension α d = 100 ± 10 degrees and declination δ d = − 24 − 13 + 12 degrees . That direction indicates an extragalactic origin for these ultrahigh-energy particles.

The first gravitational wave transient GW150914 was observed by Advanced LIGO on September 14th, 2015 at 09:50:45 Universal Time. In addition to follow-up electromagnetic observations, the detection of neutrinos will probe deeply and more on the nature of astrophysical sources, especially in the ultra-high energy regime. Neutrinos in the EeV energy range were searched in data collected at the surface detector of the Pierre Auger Observatory within ± 500 s and 1 day after the GW150914 event. No neutrino candidates were found. Based on this non-observation, we derive the first and only neutrino fluence upper limit at EeV energies for this event at 90% CL, and report constraints on existence of accretion disk around mergers.

A promising energy range to look for angular correlations between cosmic rays of extragalactic origin and their sources is at the highest energies, above a few tens of EeV (1 EeV ≡ 1018 eV). Despite the flux of these particles being extremely low, the area of ∼3000 km2 covered at the Pierre Auger Observatory, and the 17 yr data-taking period of the Phase 1 of its operations, have enabled us to measure the arrival directions of more than 2600 ultra-high-energy cosmic rays above 32 EeV. We publish this data set, the largest available at such energies from an integrated exposure of 122,000 km2 sr yr, and search it for anisotropies over the 3.4π steradians covered with the Observatory. Evidence for a deviation in excess of isotropy at intermediate angular scales, with ∼15° Gaussian spread or ∼25° top-hat radius, is obtained at the 4σ significance level for cosmic-ray energies above ∼40 EeV.