The transition from Galactic to extragalactic cosmic rays: the high-energy end of the Galactic spectrum

TL;DR

This paper reviews models of the transition from Galactic to extragalactic cosmic rays, emphasizing the importance of Galactic CR properties like maximum energy and composition in understanding this transition.

Contribution

It provides a comparative overview of different models explaining the Galactic to extragalactic CR transition and highlights the significance of Galactic CR characteristics.

Findings

Different models predict transition energies in the 4-10 EeV range.

Galactic CR maximum energy and composition are crucial for transition understanding.

High-energy Galactic CR spectrum depends on source properties and propagation effects.

Abstract

Understanding the transition from Galactic to extragalactic cosmic rays (CRs) is essential to make sense of the Local cosmic ray spectrum. Several models have been proposed to account for this transition in the 0.1 - 10 $\times 10^{18}$ eV range. For instance: ankle models, where the change from a steep Galactic component to a hard extragalactic spectrum occurs in the $4-10 \times 10^{18}$ eV region, dip models, where the interactions of CR protons with the CMB producing electron-positron pairs shape the ankle, or mixed composition models, in which extragalactic CRs are composed of nuclei of various types. In all these scenarios, the low-energy part of the transition involves the high-energy part of the Galactic component. Therefore, any information on the Galactic component, such as maximum energy, chemical composition, and spectrum after propagation, is crucial to understanding the Galactic-extragalactic transition. We briefly review the high-energy part of the CR spectrum expected from the best potential sources of Galactic CRs.