Regular magnetic black holes with anti-de Sitter cores in scalar-tensor gravity: axial stability and radial instability

Abstract

We construct static, asymptotically flat regular black holes in Einstein-frame scalar--tensor gravity, sourced by a canonical scalar coupled to magnetic nonlinear electrodynamics. Prescribing the density and scalar profiles, we reconstruct the metric, the scalar--photon coupling and the potential. Regularity requires the mass function to vanish at the centre, which fixes the ADM mass, and the central $r^2$ growth of the density to be compensated by the gradient energy of the scalar. The solutions have an anti-de Sitter core supported by a negative potential and a nearly Reissner--Nordström exterior; absence of ghosts restricts the magnetic charge to an interval that shrinks to zero at a finite core depth. Axial perturbations reduce to a symmetric two-channel master system whose positive-definite potential guarantees mode stability; spectral and time-domain quasinormal frequencies agree. Radial perturbations, by contrast, are unstable: the reconstructed potential behaves as $V\propto -{\phi }^5$ near the asymptotic vacuum, the scalar becomes tachyonic outside the horizon, and every ghost-free black hole has a growing monopole mode with $\Gamma M\approx 0.14$--$0.28$. An angular Laplacian instability develops near the centre; for black holes it may lie entirely within the inner horizon, while horizonless solutions leave it uncovered and are therefore excluded.

arXiv comment25 pages, 13 figures and 3 tables