We report a measurement of the energy spectrum of cosmic rays for energies above 2.5 × 10 18 eV based on 215,030 events recorded with zenith angles below 60°. A key feature of the work is that the estimates of the energies are independent of assumptions about the unknown hadronic physics or of the primary mass composition. The measurement is the most precise made hitherto with the accumulated exposure being so large that the measurements of the flux are dominated by systematic uncertainties except at energies above 5 × 10 19 eV. The principal conclusions are (1) The flattening of the spectrum near 5 × 10 18 eV, the so-called "ankle," is confirmed. (2) The steepening of the spectrum at around 5 × 10 19 eV is confirmed. (3) A new feature has been identified in the spectrum: in the region above the ankle the spectral index γ of the particle flux (∝ E −γ) changes from 2.51 AE 0.03 ðstatÞ AE 0.05 ðsystÞ to 3.05 AE 0.05 ðstatÞ AE 0.10 ðsystÞ before changing sharply to 5.1 AE 0.3 ðstatÞ AE 0.1 ðsystÞ above 5 × 10 19 eV. (4) No evidence for any dependence of the spectrum on declination has been found other than a mild excess from the Southern Hemisphere that is consistent with the anisotropy observed above 8 × 10 18 eV.
We report a measurement of the energy spectrum of cosmic rays above 2.5 × 10 18 eV based on 215 030 events. New results are presented: at about 1.3 × 10 19 eV, the spectral index changes from Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.
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.
We present measurements of the large-scale cosmic-ray (CR) anisotropies in R.A., using data collected by the surface detector array of the Pierre Auger Observatory over more than 14 yr. We determine the equatorial dipole component, d , through a Fourier analysis in R.A. that includes weights for each event so as to account for the main detector-induced systematic effects. For the energies at which the trigger efficiency of the array is small, the "east-west" method is employed. Besides using the data from the array with detectors separated by 1500m, we also include data from the smaller but denser subarray of detectors with 750m separation, which allows us to extend the analysis down to ∼0.03EeV. The most significant equatorial dipole amplitude obtained is that in the cumulative bin above 8EeV, = ^-+ d 6.0 0.9 1.0 %, which is inconsistent with isotropy at the 6σ level. In the bins below 8EeV, we obtain 99% CL upper bounds on d ⊥ at the level of 1%-3%. At energies below 1EeV, even though the amplitudes are not significant, the phases determined in most of the bins are not far from the R.A. of the Galactic center, at α GC =−94°, suggesting a predominantly Galactic origin for anisotropies at these energies. The reconstructed dipole phases in the energy bins above 4EeV point instead to R.A. that are almost opposite to the Galactic center one, indicative of an extragalactic CR origin.Unified Astronomy Thesaurus concepts: Cosmic rays (329); Cosmic ray sources (328); Cosmic ray astronomy (324)
More than half a century after the discovery of ultra-high energy cosmic rays (UHECRs), their origin is still an open question. The study of anisotropies in the arrival directions of such particles is an essential ingredient to solve this puzzle. We update our previous analysis of large-scale anisotropies observed by the Pierre Auger Observatory using the latest data collected before the AugerPrime upgrade. We select events with zenith angles up to 80 degrees, implying a sky coverage of 85%, and energies above 4 EeV, for which the surface detector of the Observatory is fully efficient. Dipolar and quadrupolar amplitudes are evaluated through a combined Fourier analysis of the event count rate in right ascension and azimuth. The analysis is performed in three energy bins with boundaries at 4, 8, 16 and 32 EeV and two additional cumulative bins with energies above 8 and 32 EeV. The most significant signal is a dipolar modulation in right ascension for energies above 8 EeV, as previously reported, with statistical significance of 6.6 . Additionally, we report the measurements of the angular power spectrum for the same energy bins with the same dataset.
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