Killing horizon

A Killing horizon is a surface which is part of a rotating black hole and is located outside the Schwarzschild radius. A Killing horizon, given by <math>(r-GM)^{2} = G^{2}M^{2}-J^{2}\cos^{2}\theta<math> in Boyer-Lindquist coordinates, can be characterized as the sphere where "the rotational velocity of the surrounding space" is dragged along with the velocity of light. Within this sphere the dragging is greater than the speed of light. Therefore, no observer or particle within this sphere can maintain itself in a non-rotating orbit, causing the object to become co-rotated.

Also, a Killing horizon in mathematics is a null hypersurface on which there is a null Killing vector field.