Heavy Nuclei Synthesized in Gamma-Ray Burst Outflows as the Source of UHECRs

Understand

Recent measurements by the Pierre Auger Observatory suggest that the composition of ultra-high energy cosmic rays (UHECRs) becomes dominated by heavy nuclei at high energies.

• However, until now there has been no astrophysical motivation for considering a source highly enriched in heavy elements.
• Here we demonstrate that the outflows from Gamma-Ray Bursts (GRBs) may indeed be composed primarily of nuclei with masses A ~ 40-200, which are synthesized as hot material expands away from the central engine.
• In particular, if the jet is magnetically-dominated (rather than a thermally-driven fireball) its low entropy enables heavy elements to form efficiently.