On the Origin and Survival of UHE Cosmic-Ray Nuclei in GRBs and Hypernovae

Abstract

The chemical composition of the ultra-high-energy (UHE) cosmic rays serves as an important clue for their origin. Recent measurements of the elongation rates by the Pierre Auger Observatory hint at the possible presence of heavy or intermediate mass nuclei in the UHE cosmic rays. Gamma-ray bursts (GRBs) and hypernovae have been suggested as possible sources of the UHE cosmic rays. Here we derive the constraints on the physical conditions under which UHE heavy nuclei, if they are accelerated in these sources, can survive in their intense photon fields. We find that in the GRB external shock and in the hypernova scenarios, UHE nuclei can easily survive photo-disintegration. In the GRB internal shock scenario, UHE nuclei can also survive, provided the dissipation radius and/or the bulk Lorentz factor of the relativistic outflow are relatively large, or if the low-energy self-absorption break in the photon spectrum of the prompt emission occurs above several KeV. In internal shocks and in the other scenarios, intermediate-mass UHE nuclei have a higher probability of survival against photo-disintegration than UHE heavy nuclei such as Fe.Comment: accepted by ApJ, references added, 12 pages, 4 figures, emulateapj styl