Spin-induced scalarization and magnetic fields

TL;DR

This paper investigates how strong magnetic fields near black holes influence spin-induced scalarization, finding that magnetic fields tend to oppose scalarization effects near the horizon, thus affecting the conditions for black hole scalarization.

Contribution

It provides a detailed analysis of the interplay between magnetic fields and spin-induced scalarization of black holes, revealing that magnetic fields can hinder scalarization near the horizon.

Findings

Magnetic fields near the horizon work against spin-induced scalarization.

Higher black hole spins are required for scalarization in the presence of magnetic fields.

Magnetic effects influence the horizon geometry, affecting scalarization conditions.

Abstract

In the presence of certain non-minimal couplings between a scalar field and the Gauss-Bonnet curvature invariant, Kerr black holes can scalarize, as long as they are spinning fast enough. This provides a distinctive violation of the Kerr hypothesis, occurring only for some high spin range. In this paper we assess if strong magnetic fields, that may exist in the vicinity of astrophysical black holes, could facilitate this distinctive effect, by bringing down the spin threshold for scalarization. This inquiry is motivated by the fact that self-gravitating magnetic fields, by themselves, can also promote "spin-induced" scalarization. Nonetheless, we show that in the \textit{vicinity of the horizon} the effect of the magnetic field $B$ on a black hole of mass $M$, up to $BM\lesssim 1$, works \textit{against} spin-induced scalarization, requiring a larger dimensionless spin $j$ from the black hole. A geometric interpretation for this result is suggested, in terms of the effects of rotation $vs.$ magnetic fields on the horizon geometry.