Event horizon detecting invariants

Some judiciously chosen local curvature scalars can be used to invariantly characterize event horizons of black holes in $D>3$ dimensions, but they fail for the three dimensional Bañados-Teitelboim-Zanelli (BTZ) black hole since all curvature invariants are constant. Here we provide an invariant characterization of the event horizon of the BTZ black hole using the curvature invariants of codimension one hypersurfaces instead of the full spacetime. Our method is also applicable to black holes in generic dimensions but is most efficient in three, four, and five dimensions. We give four dimensional Kerr, five dimensional Myers-Perry and three dimensional warped-anti–de Sitter, and the three dimensional asymptotically flat black holes as examples.