A new class of rotating black hole solutions endowed with a scalar hair

We present new hairy stationary and axisymmetric black hole solutions. These solutions belong to a subclass of Horndeski scalar-tensor theories, in which there is no non-minimal coupling to curvature. The full rotating solutions are obtained numerically using the Kadath library based on spectral methods. The metric does not rely on an explicit ansatz but rather on the use of a set of numerical coordinates. Black holes obtained differ from Kerr in general, and depend on a new independent parameter, a primary hair, coming from a linear time dependence in the scalar field. These solutions admit a Killing horizon like the Kerr black hole, but are non-circular, and break the zeroth law of black hole thermodynamics.