Angular Momentum and Killing Potentials

When the Penrose-Goldberg (PG) superpotential is used to compute the angular momentum of an axial symmetry, the Killing potential $Q_{(\varphi )}^{\mu\nu}$ for that symmetry is needed. Killing potentials used in the PG superpotential must satisfy Penrose's equation. It is proved for the Schwarzschild and Kerr solutions that the Penrose equation does not admit a $%Q_{(\varphi )}^{\mu\nu}$ at finite r and therefore the PG superpotential can only be used to compute angular momentum asymptotically.