Ultra-High-Energy Particle Production in Binary Mergers Endowed with Magnetic Fields

Abstract

We study the production of ultra-high-energy particles via the Ba\~nados--Silk--West (BSW) mechanism in the pre-merger phase of binary systems detected by LIGO-Virgo-KAGRA. By solving the geodesic equations for charged particles in magnetized Kerr spacetime with fields of $B \sim 10^{12}$--$10^{14}$~G, we demonstrate that collisions near the horizon can achieve center-of-mass energies $E_{\mathrm{cm}} \sim 10^{18}$-- $10^{20}$~eV, placing them firmly in the ultra-high-energy cosmic-ray (UHECR) range. We systematically explore the parameter space of merger remnants, varying black hole mass ($M \sim 20$--$150\,M_\odot$, characteristic of the binary black hole population), dimensionless spin ($\chi_f \sim 0.7$--$0.9$), magnetic field strength, and particle angular momenta. Our analysis reveals three distinct acceleration regimes: a gravity-dominated regime ($B < 10^{12}$~G) with negligible magnetic enhancement; a transition regime ($10^{12}~\text{G} \lesssim B \lesssim 10^{13}~\text{G}$) where gravitational and magnetic effects compete; and a magnetic-dominated regime ($B > 10^{13}$~G) where fields amplify collision energies by nearly an order of magnitude. For the 34 gravitational-wave events with high remnant spins ($\chi_f > 0.7$), we compute the maximum achievable energies, finding that systems with $\chi_f \gtrsim 0.85$ and $M \gtrsim 100\,M_\odot$ can reach $E_{\mathrm{max}} \sim 10^{20}$~eV. Our results establish magnetized binary mergers, particularly black hole--neutron star systems and postmerger black hole remnants formed in binary neutron star coalescences, as promising sources of UHECRs and provide quantitative predictions linking gravitational-wave observables to particle acceleration efficiency.