On the knee of Galactic cosmic rays in light of sub-TeV spectral hardenings

More than fifty years after the discovery of the knee in the cosmic ray (CR) spectra, its physical origin remains a mystery. This is partly due to the ambiguity of the energy spectra of individual components. Recently, direct measurements from several space experiments found significant spectral hardenings of CR nuclei at ∼200 GV. A joint modeling of the direct and indirect measurements may help to understand the experimental systematics and the physics of the knee. In this work, we update the phenomenological “poly-gonato” model to include the spectral hardenings, with a changing spectral index of γ + β · log E. This modification gives a reasonable description of the CR spectra in a wide energy range. However, the fits to different data sets give different results. We find that the fit to the AMS-02 and CREAM data slightly favors a relatively low energy knee of the light components. In such a case, the expected all-particle spectra under-shoot the data, which may require an extra component of CRs. The fits to AMS-02 data and the light component (H+He) data from the Tibet ASγ/ARGO-YBJ/WFCTA and KASCADE experiments give consistent results with the all-particle spectra. We further propose a possible physical realization of such a “modified poly-gonato” model of spectral hardenings by means of spatially-dependent diffusion of CRs. We find reasonably good agreement between the model predictions and the data for CR spectra, the secondary-to-primary ratios, and the amplitude of anisotropies.

Supported by National Key Research and Development Program of China (2016YFA0400200), the National Natural Science Foundation of China (11635011, 11761141001, 11663006, 11722328) and the 100 Talents program of Chinese Academy of Sciences