Electromagnetic fields on Kerr spacetime, Hertz potentials and Lorenz gauge

Understand

We review two procedures for constructing the vector potential of the electromagnetic field on Kerr spacetime, namely, the classic method of Cohen & Kegeles, yielding $A^\mu$ in a radiation gauge, and the newer method of Frolov et al., yielding $A^\mu$ in Lorenz gauge.

• We demonstrate that the vector potentials are related by straightforward gauge transformations, which we give in closed form.
• We obtain a new result for a separable Hertz potential $H^{\mu \nu}$ such that $A_{\text{lor}}^\mu = \nabla_\nu H^{\mu \nu}$.