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

Laboratoire Univers et Théories, Observatoire de Paris, Université PSL, Université Paris Cité, CNRS, F-92190 Meudon, France

^* Corresponding author: pierre.cristofari@obspm.fr

Published online: 28 April 2023

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 × 10¹⁸ eV range. For instance: ankle models, where the change from a steep Galactic component to a hard extragalactic spectrum occurs in the 4 − 10 × 10¹⁸ 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.