Origin of the spectral upturn in the cosmic-rayC/FeandO/Feratios

Abstract

The observed spectrum of Galactic cosmic rays has several exciting features such as the rise in the positron fraction above $\ensuremath{\sim}10\text{ }\text{ }\mathrm{GeV}$ of energy and the spectral hardening of protons and helium at $\ensuremath{\gtrsim}300\text{ }\text{ }\mathrm{GeV}/\mathrm{n}\mathrm{ucleon}$ of energy. The ATIC-2 experiment has recently reported an unexpected spectral upturn in the elemental ratios involving iron, such as the $\mathrm{C}/\mathrm{Fe}$ or $\mathrm{O}/\mathrm{Fe}$ ratios, at energy $\ensuremath{\gtrsim}50\text{ }\text{ }\mathrm{GeV}$ 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 $\mathrm{C}/\mathrm{Fe}$ and $\mathrm{O}/\mathrm{Fe}$ ratios in good accordance with the ATIC-2 data.