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Volume 358 - 36th International Cosmic Ray Conference (ICRC2019) - Invited Talks
News from Cosmic Ray Air Showers (Cosmic Ray Indirect - CRI Rapporteur)
F. Schroeder
Full text: pdf
Pre-published on: 2019 October 08
Published on:
Abstract
This rapport summarizes the cosmic-ray indirect (CRI) session of the 36th ICRC conference. Updated measurements from several air-shower arrays with higher precision were shown leading to the discovery of new features in the energy spectrum: HAWC measures a softening of the light component (p+He) around $10^{13.5}\,$eV; measurements of the Pierre Auger Observatory show that the second knee is a smooth feature extending at least over the range of $100-200\,$PeV and that the energy spectrum between the ankle and the cut-off cannot be described by a simple broken power law. Measurements of the mass composition confirm that the composition is a varying mixture of protons and nuclei at least up to several $10\,$EeV. Hadronic interaction models constitute a significant uncertainty in the interpretation of measurements, but a joint effort of several collaborations helps to better assess their deficiencies, e.g., by quantifying the muon deficit in the models over the shower energy. Still, general trends in the average mass composition over energy are consistent for all state-of-the-art models. Anisotropy measurements with higher precision generally confirm earlier results, too. A change of the amplitude and phase of the equatorial dipole provides another indication that in the energy range between the second knee and the ankle there likely is a transition from Galactic to extragalactic sources. However, neither the most energetic Galatic nor the extragalactic sources have been discovered, yet, which remains a primary science goal of the field. Next to more exposure, an increase of measurement accuracy and decrease of systematic uncertainties will provide future progress. Therefore, it is particularly exciting that new experiments are built and existing experiments upgraded to increase the accuracy for the measurement of the energy and mass composition, e.g., by combining radio antennas with particle detectors. Last but not least, there is a trend that experiments are designed such that they can target cosmic rays, photons, and neutrinos at the same time, which will facilitate multi-messenger astrophysics at the highest energies.
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