ABSTRACT:
A century after the discovery of cosmic rays, their astrophysical origins remain unconfirmed and intensely studied. Cosmic rays of the highest energies cannot feasibly be confined within our Galaxy and is thought to originate from extragalactic sources. While several source candidates have been proposed, disentangling them is generally complicated by the fact that charged cosmic rays bend in the magnetic universe. New observational results on the composition may provide new insights: it has recently been reported that the composition of cosmic rays at the highest energies become increasingly heavy (Fe-like). I will review the composition claims and discuss implications for source candidates. In particular I will focus on the potential that gamma-ray bursts provide a uniquely heavy-nuclei dominated source.