Escape model for Galactic cosmic rays and an early extragalactic transition

TL;DR

This paper presents a model explaining the cosmic ray knee as energy-dependent leakage from the Milky Way, fitting existing data and predicting a transition to extragalactic cosmic rays around 2×10^18 eV, with implications for cosmic ray composition and anisotropy.

Contribution

It introduces a comprehensive escape model that accounts for the cosmic ray knee and the Galactic to extragalactic transition, fitting multiple observational datasets with a unified framework.

Findings

CR escape time exhibits a knee-like structure around 10^15 eV.

Model fits data from KASCADE and KASCADE-Grande with simple injection spectra.

Predicts strong variation in the CR dipole phase between 10^17 and 3×10^18 eV.

Abstract

We show that the cosmic ray (CR) knee can be entirely explained by energy-dependent CR leakage from the Milky Way, with an excellent fit to all existing data. We test this hypothesis calculating the trajectories of individual CRs in the Galactic magnetic field. We find that the CR escape time $\tau_{\rm esc}(E)$ exhibits a knee-like structure around $E/Z={\rm few}\times 10^{15}$ eV for small coherence lengths and strengths of the turbulent magnetic field. The resulting intensities for different groups of nuclei are consistent with the ones determined by KASCADE and KASCADE-Grande, using simple power-laws as injection spectra. The transition from Galactic to extragalactic CRs is terminated at $\approx 2\times 10^{18}$ eV, while extragalactic CRs contribute sizeable to the subdominant proton flux already for $\gtrsim 2\times 10^{16}$ eV. The natural source of extragalactic CRs in the intermediate energy region up to the ankle are in this model normal and starburst galaxies. The escape model provides a good fit to $\ln(A)$ data; it predicts that the phase of the CR dipole varies strongly in the energy range between $1\times 10^{17}$ and $3\times 10^{18}$ eV, while our estimate for the dipole magnitude is consistent with observations.