Hairy black holes by gravitational decoupling

Black holes with hair represented by generic fields surrounding the central source of the vacuum Schwarzschild metric are examined under the minimal set of requirements consisting of (i) the existence of a well defined event horizon, namely, we require the Killing horizon coincides with the causal horizon and (ii) the strong or dominant energy condition for the hair outside the horizon. We develop our analysis by means of the gravitational decoupling approach. We find that trivial deformations of the seed Schwarzschild vacuum preserve the energy conditions and provide a new mechanism to evade the no-hair theorem based on a primary hair associated with the charge generating these transformations. Under the above conditions (i) and (ii), this charge consistently increases the entropy from the minimum value given by the Schwarzschild geometry. As a direct application, we find a non-trivial extension of the Reissner–Nordström black hole showing a surprisingly simple horizon. Finally, the non-linear electrodynamics generating this new solution is fully specified.