Investigating the signs of evolutionary characteristics in the energy spectrum of shock wave acceleration

TL;DR

This paper analyzes cosmic-ray energy spectra to identify variations in spectral indices across elements, challenging ideal shock acceleration predictions and suggesting A/Z-dependent effects.

Contribution

It constrains cosmic-ray spectral indices using AMS-02 and DAMPE data within a spatially dependent propagation model, revealing element-dependent spectral variations.

Findings

Spectral indices correlate positively with atomic and mass numbers for A/Z=2 elements.

Ni and Zn spectra are predicted to match Fe spectra, with softer injection spectra.

Future observations are expected to confirm these spectral variations.

Abstract

Under ideal conditions, the theory of shock acceleration for cosmic rays predicts that different elements should exhibit strictly identical spectral indices when accelerated to the same rigidity (R). However, recent high-precision measurements of elemental energy spectra have definitively established the existence of variations in spectral indices across different elements. This study constrains the spectral indices of cosmic-ray elements using AMS-02 and DAMPE observations within the Spatially Dependent Propagation (SDP) model. For elements with A/Z = 2, spectral indices shows significant positive correlations with both atomic number Z and mass number A, likely due to A or Z-dependent fragmentation cross-sections. Predictions indicate that the observed spectra of Ni and Zn will align with the Fe spectrum, while their injection spectra will exhibit slightly softer spectral indices compared to Fe. Future observations from AMS-02, DAMPE and HERD are expected to verify these findings, while theoretical models are needed to systematically explain this phenomenon.