 Bifurcate Killing horizon

A bifurcate Killing horizon consists of two intersecting null Killing horizons of the same Killing vector field, which vanishes on their spacelike intersection, the bifurcation surface. In regular Kruskal coordinates the horizons are $U=0$ and $V=0$, and their intersection is $U=V=0$. The displayed field is tangent and normal to each horizon branch and zero on the intersection.