Novel Hairy Black Hole Solutions in Einstein-Maxwell-Gauss-Bonnet-Scalar Theory

Abstract: It has been previously shown that a Gauss-Bonnet term non-minimally coupled to a scalar field produces a scalarised black hole solution, which can be considered as having secondary scalar hair, parametrised in terms of the black hole's mass and charge. In this paper we extend a previously investigated linear coupling of the form $f(\phi)=\phi$ to a non-minimally coupled Maxwell term, with the form $\frac{1}{8}F_{\mu\nu}F^{\mu\nu}+\beta\phi F_{\mu\nu}F^{\mu\nu}$. By using numerical methods the solutions to the full differential equations are found, as well as a perturbative expansion in the $r\rightarrow\infty$ limit and a perturbative expansion in couplings parameters such as $\beta$. These solutions describe scalarised black holes with modified electric field which have dependence not only on the electric charge of the black hole, but also the value of the non-minimal coupling constant and the horizon radius or mass of the black hole. We also discuss the bounds imposed on the parameters of the black hole by the reality condition of the solution, giving some explicit numerical bounds.