Ultrahigh Energy Cosmic Ray Production in Binary Neutron Star Mergers

Abstract

Having previously argued that binary neutron star mergers are the principle source of ultrahigh energy cosmic rays~\citep{fBNS-prl25}, we exploit here the highly constrained initial conditions to make quantitative predictions for the cutoff energy of various nuclei. UHECRs heavier than helium are accelerated in the magnetized turbulent outflow outside the jets to a rigidity $\mathcal{R}_{\rm cut} \equiv E_{\rm cut}/eZ \approx 6-9$ EV, consistent with the measured value $\mathcal{R}_{\rm cut} = 6.3^{+6.3}_{-2.3}\,$EV from fitting data. This agreement strengthens the case that BNS mergers are the main site of UHECR production. The jets may accelerate protons and/or helium to cutoff energies $\approx 11.5$ and $\approx 35$ EeV, respectively. Such a jet component and its spallation products could explain the indication of a secondary light population at higher energy found in the analysis of~\citet{muf19}. The relative abundances of different elements and the total energy in UHECRs per merger event will become calculable, pending advances in our understanding of the mechanism of ion uptake into the acceleration process and input from nuclear physics experiments. This scenario implies that each neutrino above 1 PeV is co-directional with a gravitational wave arriving $\approx 1$ day earlier, and that the highest energy UHECRs have masses heavier than iron.