Spin-induced black hole scalarization in Einstein-scalar-Gauss-Bonnet theory

TL;DR

This paper constructs and analyzes spinning black hole solutions with scalarization induced by the Gauss-Bonnet term, revealing new stable configurations that can violate the Kerr bound and are entropically favored.

Contribution

It introduces a new class of spin-induced scalarized black hole solutions in Einstein-scalar-Gauss-Bonnet theory, exploring their properties and stability.

Findings

Scalarized black holes can violate the Kerr rotation bound.

Solutions with scalarization are entropically favored over Kerr black holes.

Identification of critical solutions where metric and scalar field expansions break down.

Abstract

We construct black hole solutions with spin-induced scalarization in a class of models where a scalar field is quadratically coupled to the topological Gauss-Bonnet term. Starting from the tachyonically unstable Kerr solutions, we obtain families of scalarized black holes such that the scalar field has either even or odd parity, and we investigate their domain of existence. The scalarized black holes can violate the Kerr rotation bound. We identify "critical" families of scalarized black hole solutions such that the expansion of the metric functions and of the scalar field at the horizon no longer allows for real coefficients. For the quadratic coupling considered here, solutions with spin-induced scalarization are entropically favored over Kerr solutions with the same mass and angular momentum.