Spacetimes admitting isolated horizons

We characterize a general solution to the vacuum Einstein equations which admits isolated horizons. We show that it is a nonlinear superposition (in a precise sense) of the Schwarzschild metric with a certain free data set propagating tangentially to the horizon. This proves Ashtekar's conjecture about the structure of spacetime near the isolated horizon. The same superposition method applied to the Kerr metric gives another class of vacuum solutions admitting isolated horizons. More generally, a vacuum spacetime admitting any null, non-expanding, shear-free surface is characterized. The results are applied to show that, generically, the non-rotating isolated horizon does not admit a Killing vector field and a spacetime is nor spherically symmetric near a symmetric horizon.