Cosmic Ray Origin: Lessons from Ultra-High-Energy Cosmic Rays and the Galactic/Extragalactic Transition

TL;DR

This paper discusses the origin of Galactic cosmic rays, suggesting the transition to extragalactic sources occurs at the ankle energy, and explores superbubbles as potential cosmic-ray accelerators beyond standard supernova remnants.

Contribution

It proposes that superbubbles may be the primary sites of cosmic-ray acceleration, challenging the traditional supernova remnant model and aligning with recent high-energy cosmic-ray data.

Findings

Transition between Galactic and extragalactic cosmic rays occurs at the ankle.

Galactic protons likely reach energies around 10^{17} eV, challenging standard models.

Superbubbles may significantly contribute to cosmic-ray acceleration.

Abstract

We examine the question of the origin of the Galactic cosmic-rays (GCRs) in the light of the data available at the highest energy end of the spectrum. We argue that the data of the Pierre Auger Observatory and of the KASCADE-Grande experiment suggest that the transition between the Galactic and the extragalactic components takes place at the energy of the ankle in the all-particle cosmic-ray spectrum, and at an energy of the order of $10^{17}$ eV for protons. Such a high energy for Galactic protons appears difficult to reconcile with the general view that GCRs are accelerated by the standard diffusive shock acceleration process at the forward shock of individual supernova remnants (SNRs). We also review various difficulties of the standard SNR-GCR connection, related to the evolution of the light element abundances and to significant isotopic anomalies. We point out that most of the power injected by the supernovae in the Galaxy is actually released inside superbubbles, which may thus play an important role in the origin of cosmic-rays, and could solve some persistent problems of the standard SNR-GCR scenario in a rather natural way.