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

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 about 10 12 -- 10 14 G, we demonstrate that collisions near the horizon can achieve center-of-mass energies E cm about 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 about 20 -- 150,M, characteristic of the binary black hole population), dimensionless spin (chi f about 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 G B 10 13 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 0.85 and M 100,M can reach E max about 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.