33RD INTERNATIONAL COSMIC RAY CONFERENCE, RIO DE JANEIRO 2013 THE ASTROPARTICLE PHYSICS CONFERENCE Ultra-high energy cosmic rays: A review of the galactic-source conjecture

Abstract: The idea that most ultra-high energy cosmic rays (UHECR) originate beyond the galaxy has been a common conjecture in the literature for some time. This extragalactic-source conjecture has been supported in part by the apparent difficulty of the galactic-source conjecture, i.e. the lack of objects in the galaxy that can accelerate 1020 eV protons and the lack of cosmic-ray (CR) anisotropy associated with the galaxy. However, if protons are sub-dominant above the ankle energy and instead intermediate and iron-like nuclei dominate the CR flux, much of the difficulty of the galactic-source conjecture is eliminated. In this contribution we review the plausibility of the galactic-source conjecture. We show that it is plausible that UHECR near the ankle gyrate in the disk of the galaxy and may scatter off of structures in the magnetic field effectively isotropizing UHECR to a level consistent with current observations. We review possible source candidates including trans-relativistic supernova and young, fast rotating pulsars, and discuss possible mechanisms for the ankle and the features in the cosmic-ray energy spectrum. We suggest that UHECR may be accelerated episodically in the galaxy with the period between acceleration events being ∼ 104 yrs