Slowly rotating black holes in the novel Einstein–Maxwell-scalar theory

Abstract

We investigate a slowly rotating black hole solution in a novel Einstein–Maxwell-scalar theory, which is prompted by the classification of general Einstein–Maxwell-scalar theory. The gyromagnetic ratio of this black hole is calculated, and it increases as the second free parameter $$\beta $$ β increases, but decreases with the increasing parameter $$\gamma \equiv \frac{2 \alpha ^{2}}{1+\alpha ^2}$$ γ ≡ 2 α 2 1 + α 2 . In the Einstein–Maxwell-dilaton (EMD) theory, the parameter $$\beta $$ β vanishes but the free parameter $$\alpha $$ α governing the strength of the coupling between the dilaton and the Maxwell field remains. The gyromagnetic ratio is always less than 2, the well-known value for a Kerr–Newman (KN) black hole as well as for a Dirac electron. Scalar hairs reduce the magnetic dipole moment in dilaton theory, resulting in a drop in the gyromagnetic ratio. However, we find that the gyromagnetic ratio of two can be realized in this Einstein–Maxwell-scalar theory by increasing $$\beta $$ β and the charge-to-mass ratio Q/M simultaneously (recall that the gyromagnetic ratio of KN black holes is independent of Q/M). The same situation also applies to the angular velocity of a locally non-rotating observer. Moreover, we analyze the period correction for circular orbits in terms of charge-to-mass ratio, as well as the correction of the radius of the innermost stable circular orbits. It is found the correction increases with $$\beta $$ β but decreases with Q/M. Finally, the total radiative efficiency is investigated, and it can vanish once the effect of rotation is considered.

Adiabatic accretion onto black holes in Einstein-Maxwell-scalar theory

“…Einstein-Maxwell-scalar theory, which arises as a low-energy effective theory in string theory, extends the Einstein-Maxwell theory by including the kinetic term associated to a background scalar field and the coupling to the Maxwell term [59,60], with its special case known as the Einstein-Maxwell-dilaton (EMD) theory [61,62]. The action of EMS theory has the following form:…”

Causal structure and the geodesics in the hairy extension of the Bertotti-Robinson spacetime

“…Among the non-asymptotically flat solutions, some are asymptotically de-Sitter or anti-de Sitter [44][45][46][47][48][49][50][51][52][53][54]which have applications in the anti-de Sitter, conformal field theory correspondence (AdS/CFT) in the context of the unification of the quantum field theory and gravitation [55]. We note that EMD is not the only extension of the Einstein-Maxwell (EM) theory and the coupling of gravity to scalar fields through Maxwell's gauge i.e., Einstein-Maxwell-Scalar (EMS) theory, has also received great attention in the literature [56][57][58][59][60][61][62][63][64][65]. For a good classification of EMS theory, we refer to a recent paper by Astefanesei et al [66].…”