Rare Transients in Nearby Galaxies Explain Ultra-high-energy Cosmic Rays

The origin of ultra-high-energy cosmic rays remains one of the central open questions in astroparticle physics. Recent measurements reveal anisotropies in arrival directions, a rigidity-dependent composition dominated by intermediate-mass nuclei, and striking hemispheric differences in the energy spectra. Here we show that \emph{rare transients in nearby galaxies} can naturally account for these features. In our fiducial neutron-star merger model, the cosmic ray flux above $25$~EeV is dominated by ten nearby galaxies within $8$Mpc. This accounts for the observed hotspots: seven of the ten brightest galaxies coincide with reported excess regions, a chance probability of $p\sim 0.001$. Nearby transients can also explain the spectral excess of TA over Auger and modify the rigidity–aligned succession of isotopes.