Scalarization of charged Taub-NUT black hole and the entropy bound

TL;DR

This paper explores the spontaneous scalarization of charged Taub-NUT black holes in Einstein-Maxwell-scalar-Gauss-Bonnet gravity, revealing new hairy solutions with higher entropy and universal features at bifurcation points.

Contribution

It introduces a new class of scalarized charged Taub-NUT black holes and analyzes their properties, including entropy behavior, within a unified theoretical framework.

Findings

Scalarized solutions have higher entropy than scalar-free ones.

Entropy peaks at bifurcation points, indicating phase transitions.

Maximum entropy values are universal for fixed electric charge.

Abstract

We investigate the spontaneous scalarization of charged Taub-NUT black holes within the framework of Einstein-Maxwell-scalar-Gauss-Bonnet gravity. By selecting a suitable coupling function, the theory admits the analytic charged Taub-NUT geometry as a solution. We demonstrate that this scalar-free background becomes unstable within specific parameter regimes, leading to the bifurcation of a new branch of hairy charged Taub-NUT black holes. These solutions are characterized by a two-dimensional parameter space spanned by the electric charge and the NUT parameter. We conduct a systematic study of their properties, specifically the scalar charge, temperature, and entropy. Our analysis reveals that the entropy of the scalarized solutions exhibits particularly compelling features. Two universal characteristics emerge: first, the entropy of the hairy black hole is strictly greater than that of its scalar-free counterpart; second, the entropy reaches a local maximum precisely at the bifurcation point. Notably, when the electric charge is fixed, this maximum entropy value remains universal across a specific range of the mass parameter.