Can binary mergers produce maximally spinning black holes?

TL;DR

This paper predicts the final spin of black holes resulting from binary mergers, showing that maximally spinning black holes can be produced, especially with aligned spins and small mass ratios, using extrapolated formulas.

Contribution

It introduces new formulas for final black hole spin and mass based on extrapolation from test-particle limits, improving predictions for aligned and anti-aligned mergers.

Findings

Predicted final spin for equal-mass non-spinning binaries with better than 1% accuracy.

Equal-mass maximally spinning aligned mergers produce nearly maximally spinning black holes.

Black holes can be spun up by aligned mergers if the mass ratio is sufficiently small.

Abstract

Gravitational waves carry away both energy and angular momentum as binary black holes inspiral and merge. The relative efficiency with which they are radiated determines whether the final black hole of mass $M_f$ and spin $S_f$ saturates the Kerr limit ($\chi_f \equiv S_f/M_f^2 \leq 1$). Extrapolating from the test-particle limit, we propose expressions for $S_f$ and $M_f$ for mergers with initial spins aligned or anti-aligned with the orbital angular momentum. We predict the the final spin at plunge for equal-mass non-spinning binaries to better than 1%, and that equal-mass maximally spinning aligned mergers lead to nearly maximally spinning final black holes ($\chi_f \simeq 0.9988$). We also find black holes can always be spun up by aligned mergers provided the mass ratio is small enough.