Ultraheavy Ultrahigh-Energy Cosmic Rays

TL;DR

This paper explores the propagation and origins of ultraheavy nuclei as ultrahigh-energy cosmic rays, suggesting they could explain observed phenomena and align with current astrophysical source models.

Contribution

It provides the first constraints on ultraheavy UHECR sources and predicts observable differences in shower depth for future experiments.

Findings

UH nuclei have longer energy loss lengths than protons at high energies.

Current data are compatible with UHECRs from collapsars and neutron star mergers.

Future measurements can test the predicted depth of shower maximum for UH nuclei.

Abstract

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, including the Amaterasu particle, may be UH-UHECRs. For the first time, we derive constraints on the contribution of UH-UHECR sources, and find that the current data are consistent with energy generation rate densities of UHECRs from collapsars and neutron star mergers. Our model predicts that the mean value of the depth of shower maximum is lower than that for iron nuclei beyond 100 EeV, which can be tested with future composition measurements, e.g., AugerPrime and the Global Cosmic Ray Observatory. In addition, the spectral tension between the Telescope Array (TA) and the Pierre Auger Observatory can be alleviated by considering the enhanced contribution of UHECRs -- including UH nuclei -- from a nearby transient.