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

In a previous paper, arXiv:2411.11960 [gr-qc], 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 ${\mathrm{{\rm Y}}}_f=0.97$, and observe that the maximum ${\mathrm{{\rm Y}}}_f$ increases monotonically with $\lambda$ for a fixed initial Lorentz factor. We find that binaries with larger $\lambda$ radiate less total energy despite having stronger electromagnetic emission. The maximum energy radiated by a binary in our study is $31\mathrm{\%}$ of its gravitational mass. Increasing $\lambda$ has little effect on the maximum angular momentum radiated, which was $\approx 72\mathrm{\%}$ of the spacetime total angular momentum for each $\lambda$ explored here. Lastly, we provide additional evidence for the universality with the irreducible mass that we discovered in arXiv:2411.11960 [gr-qc]. The black hole horizon areal radius determines a fundamental, gauge-invariant length scale governing BH interactions near the scattering threshold.