Ultraheavy Nuclei as Ultrahigh-Energy Cosmic Rays: Constraints from Collapsars and neutron star mergers

We investigate the propagation of ultraheavy (UH) nuclei as ultrahigh-energy cosmic rays (UHECRs). We show that their energy loss lengths at $\lesssim300$~EeV are significantly longer than those of protons and intermediate-mass nuclei, and that the highest-energy cosmic rays with energies beyond $\sim100$~EeV, may originate from such UH-UHECRs. We derive constraints on the contribution of UH-UHECR sources, and the allowed energy generation rate densities are consistent with those of collapsars and compact binary mergers. A nearby transient, e.g., a low-luminosity gamma-ray burst (LL GRB), may give a special contribution to the observed UHECR flux, and we explore the possibility of simultaneously explaining both the Auger and TA spectra. These models predict a measurable shift toward heavier compositions above 100~EeV, testable with next-generation UHECR observatories.