Charge-dependent spectral softenings of primary cosmic-rays below the knee

TL;DR

This study presents the first direct measurements of cosmic-ray spectra for carbon, oxygen, and iron, revealing a universal spectral softening at ~15 TV rigidity, challenging mass-dependent models.

Contribution

It provides the first direct detection of charge-dependent spectral softening in cosmic rays, supporting rigidity-dependent effects over mass-dependent ones.

Findings

Spectral softening detected in carbon, oxygen, and iron spectra.

Softening occurs universally at ~15 TV rigidity.

Mass-dependent softening is statistically rejected.

Abstract

In most particle acceleration or propagation theories, the characteristic features of the cosmic ray spectra due to acceleration limits or propagation phase changes are charge dependent. Alternatively, the interaction scenario would expect mass dependent spectral features in general. The observational verification of which relation takes effect in nature is still lack due to the difficulty of measuring the spectra of individual particles up to very high energies. Here we report direct measurements of the carbon, oxygen, and iron spectra from ~20 gigavolts to ~100 teravolts (~60 teravolts for iron) with 9 years of on-orbit data collected by the Dark Matter Particle Explorer. Distinct spectral softenings have been directly detected in these spectra for the first time. Combined with the updated proton and helium spectra, the spectral softening appears universally at a rigidity of ~15 teravolts. A nuclei mass dependent softening is rejected at a confidence level of >99.999%. Possible interpretations of these results, including a nearby cosmic ray source and other models such as the propagation effect, are discussed.