The dyonic Kerr-Schild ansatz

TL;DR

This paper extends the Kerr-Schild ansatz to include both electric and magnetic fields in a unified geometric framework, leading to a clear derivation of dyonic solutions like Kerr-Newman.

Contribution

It introduces a geometrically motivated dyonic vector potential within the Kerr-Schild ansatz, unifying electric and magnetic sectors without duality rotations.

Findings

Derived a dyonic Kerr-Newman solution within the extended ansatz.

Showed the circularity theorem constrains electric-magnetic splitting.

Extended the formalism to (A)dS spacetimes.

Abstract

We develop a geometric extension of the Kerr-Schild ansatz that incorporates both electric and magnetic sectors of the Maxwell field in a unified framework, without resorting to duality rotations. We start observing that the known purely electric solution satisfies Maxwell's equations due to a closedness condition obeyed by the Kerr-Schild null congruence. From the associated local exactness property, we construct a new one-form naturally linked to the congruence as a sort of Poincar\'e dualization. This leads us to propose a geometrically motivated dyonic vector potential within the Kerr-Schild ansatz, defined as a superposition of an electric contribution along the congruence and a magnetic one that aligns to the dualized one-form. We then show that for a stationary and axisymmetric Kerr-Schild ansatz, the electrovac circularity theorem uniquely constrains not only the scalar profile of the metric, but also those associated to the electric-magnetic splitting of the gauge field. The resulting formalism provides a transparent derivation of the dyonic Kerr-Newman solution and extends naturally to the (A)dS case, highlighting the intrinsic interplay between geometry and matter in a Kerr-Schild setting.