Origin of the spectral upturn in the cosmic-ray C/Fe and O/Fe ratios

Abstract

The observed spectrum of Galactic cosmic rays has several exciting features such as the rise in the positron fraction above ~10 GeV of energy and the spectral hardening of protons and helium at ~300 GeV/nucleon of energy. The ATIC-2 experiment has recently reported an unexpected spectral upturn in the elemental ratios involving iron, such as the C/Fe or O/Fe ratios, at energy above 50 GeV per nucleon. It is recognized that the observed positron excess can be explained by pion production processes during diffusive shock acceleration of cosmic-ray hadrons in nearby sources. Recently, it was suggested that a scenario with nearby source dominating the GeV-TeV spectrum may be connected with the change of slope observed in protons and nuclei, which would be interpreted as a flux transition between the local component and the large-scale distribution of Galactic sources. Here I show that, under a two-component scenario with nearby source, the shape of the spectral transition is expected to be slightly different for heavy nuclei, such as iron, because their propagation range is spatially limited by inelastic collisions with the interstellar matter. This enables a prediction for the primary/primary ratios between light and heavy nuclei. From this effect, a spectral upturn is predicted in the C/Fe and O/Fe ratios in good accordance with the ATIC-2 data.

5 pages, 3 figures

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• Imprints of Cosmic Rays in Multifrequency Observations of the Interstellar Emission
• Northern sky Galactic Cosmic Ray anisotropy between 10-1000 TeV with the Tibet Air Shower Array
• Observation of Spectral Structures in the Flux of Cosmic-Ray Protons from 50 GeV to 60 TeV with the Calorimetric Electron Telescope on the International Space Station
• Solar and nuclear physics uncertainties in cosmic-ray propagation
• Direct Measurement of the Cosmic-Ray Helium Spectrum from 40 GeV to 250 TeV with the Calorimetric Electron Telescope on the International Space Station
• Origin of the Proton-to-Helium Ratio Anomaly in Cosmic Rays
• Limits on the Dark Matter from AMS-02 antiproton and positron fraction data
• On the knee of Galactic cosmic rays in light of sub-TeV spectral hardenings
• Testing universality of cosmic-ray acceleration with proton/helium data from AMS and Voyager-1