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Ultra Heavy Cosmic Rays from Magnetars

Matter ejected from the neutron star crust during a magnetar giant flare will undergo $r$-process nucleosynthesis during decompression. Ultra heavy ions ($Z \gg 26$) can be accelerated to cosmic ray energies by the reverse shock as the ejecta decelerates by interacting with the ambient environment. We investigate the contribution of magnetars to the local ultra heavy cosmic ray flux using semi-analytic Galactic transport calculations, demonstrating that they may be significant contributors throughout Galactic history depending on the giant flare rate and ion acceleration efficiency. Although neutron star mergers inject orders of magnitude more energy into cosmic rays, they rarely occur within the spallation-limited propagation horizon for ultra heavy species, reducing their local contributions. As compared to lighter nuclei which are dominantly accelerated by supernovae, the SuperTIGER experiment has presented tentative evidence for a distinct contribution to the cosmic ray abundances near and above the first $r$-process peak ($Z \approx 35\text{--}56$). We argue that current abundance data are consistent with either a magnetar giant flare or neutron star merger origin for these species. Measurements with single element resolution through the third $r$-process peak, expected from the upcoming TIGERISS experiment, may discriminate between these sources for the heaviest cosmic rays.