Radial perturbations of scalar-Gauss-Bonnet black holes beyond spontaneous scalarization

TL;DR

This paper investigates the stability of nonlinearly scalarized black holes in scalar-Gauss-Bonnet gravity, showing that certain phases can be stable and hyperbolic, thus viable as astrophysical objects.

Contribution

It introduces a new form of scalarization beyond the standard one and analyzes the stability and hyperbolicity of the resulting black hole solutions.

Findings

Some nonlinearly scalarized black hole phases are stable for large masses.

These black holes can maintain hyperbolicity, making them viable astrophysical candidates.

The study extends understanding of black hole scalarization beyond spontaneous scalarization.

Abstract

Spontaneous scalarization of black holes in scalar-Gauss-Bonnet (sGB) gravity is a very interesting phenomenon allowing black holes to circumvent the no-hair theorem and acquire scalar hair while leaving the weak field regime of the theory practically unaltered. It was recently shown that if we allow for a different form of the coupling between the scalar field and the Gauss-Bonnet invariant, a new type of scalarization is possible beyond the standard one that can be excited only nonlinearly. In this case the spectrum of black hole solutions can be more complicated, naturally opening the question about their stability. The goal of the present paper is to study this question including the possible loss of hyperbolicity of the radial perturbation equation. We show that one of the nonlinearly scalarized phases of the Schwarzschild black hole can be stable and hyperbolic for large enough black hole masses. These results establish the studied hairy black holes as viable astrophysical candidates.