New Asymptotically Flat Einstein-Maxwell Instantons

We disprove the Euclidean Einstein-Maxwell black hole uniqueness conjecture, and thus demonstrate that the semiclassical properties of coupled gravitational and electromagnetic fields are more subtle than expected from Lorentzian general relativity, where the Kerr-Newman family of metrics yields the most general stationary and asymptotically flat black holes with a single event horizon. This is achieved by an explicit construction of a new three-parameter family of asymptotically flat Einstein-Maxwell instantons. These solutions are toric, regular, and free of conical and orbifold singularities on the manifold M=CP^{2}\S^{1}. In the case of vanishing charge, these instantons reduce to the Chen-Teo Ricci-flat instantons.