Conserved currents for a Kerr black hole and orthogonality of quasinormal modes

Using a symmetry of the Kerr spacetime, the authors determine an original bilinear form for perturbations of the Weyl scalars. They show that upon choosing appropriate integration contours, this form can also be used on quasi-normal mode data. This manuscript provides a formalism for studying ringdown beyond the leading order, with potential application to numerical relativity and gravitational wave observation.