Charged spherically symmetric black holes in scalar-tensor Gauss-Bonnet gravity

We derive a novel class of four-dimensional black hole (BH) solutions in Gauss-Bonnet (GB) gravity coupled with a scalar field in presence of Maxwell electrodynamics. In order to derive such solutions, we assume the ansatz gtt not equal grr-1 for metric potentials. Due to the choice of the ansatz of the metric, the Reissner Nordstrom gauge potential cannot be recovered because of the presence of higher-order terms which are not allowed to be vanishing. Moreover, the scalar field is not allowed to vanish. If it vanishes, a function of the solution results undefined. Furthermore, it is possible to show that the electric field is of higher-order in the monopole expansion: this fact explicitly comes from the contribution of the scalar field. Therefore, we can conclude that the GB scalar field acts as non-linear electrodynamics creating monopoles, quadrupoles, etc in the metric potentials. We compute the invariants associated with the BHs and show that, when compared to Schwarzschild or Reissner-Nordstrom space-times, they have a soft singularity. Also, it is possible to demonstrate that these BHs give rise to three horizons in AdS space-time and two horizons in dS space-time. Finally, thermodynamic quantities can be derived and we show that the solution can be stable or unstable depending on a critical value of the temperature.