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

TL;DR

This paper investigates the properties and energetics of near-extremal charged black hole merger remnants through simulations, revealing universal behaviors and quantifying energy and angular momentum radiated during high-energy interactions.

Contribution

It provides new simulation results on merger remnants with high Kerr-Newman parameters and demonstrates universality with the irreducible mass in charged black hole interactions.

Findings

Merger remnants can reach Kerr-Newman parameter $ = 0.97.

Maximum energy radiated is 31% of the total mass.

Universality with the irreducible mass governs black hole interactions.

Abstract

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 $\Upsilon_f = 0.97$, and observe that the maximum $\Upsilon_f$ 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 $31\%$ of its gravitational mass. Increasing $\lambda$ has little effect on the maximum angular momentum radiated, which was $\approx 72\%$ 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.