High-energy interactions of charged black holes in full general relativity. II. Near-extremal merger remnants and universality with the irreducible mass

In a preceding paper [M A. M. Smith, V. Paschalidis, and G. Bozzola, High-energy interactions of charged black holes in full general relativity. I. Zoom-whirl orbits and universality with the irreducible mass, Phys. Rev. D 111, 104031 (2025)PRVDAQ2470-001010.1103/PhysRevD.111.104031], we initiated a study of high-energy interactions of charged binary black holes near the scattering threshold, focusing on zoom-whirl orbits. In this second paper in our series, we focus on merger remnant properties and energetics with new simulations of equal-mass, equal-charge, nonspinning binary black holes with variable impact parameter. We find near-extremal merger remnants with Kerr-Newman parameter reaching ϒf=0.97, and find evidence that the maximum ϒf increases monotonically with λ for a fixed initial Lorentz factor. We find that binaries with larger λ radiate less total energy despite having stronger electromagnetic emission. The maximum energy radiated by a binary in our study is 31% of its gravitational mass. Increasing λ has little effect on the maximum angular momentum radiated, which was ≈72% of the spacetime total angular momentum for each λ explored here. Lastly, we provide additional evidence for the universality with the irreducible mass at fixed initial linear momenta that we discovered in [M A. M. Smith, V. Paschalidis, and G. Bozzola, High-energy interactions of charged black holes in full general relativity I: Zoom-whirl orbits and universality with the irreducible mass, Phys. Rev. D 111, 104031 (2025)]. The black hole horizon areal radius may determine a fundamental, gauge-invariant length scale governing black hole interactions near the scattering threshold.