Spin-induced scalarization and magnetic fields

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.