We examine hidden symmetry and its relation to the separability of the Maxwell equation on the Wahlquist spacetime. After seeing that the Wahlquist spacetime is a type-D spacetime whose repeated principal null directions are shear-free and geodesic, we show that the spacetime admits three gauged conformal Killing-Yano (GCKY) tensors which are in a relation with torsional conformal Killing-Yano tensors. As a by-product, we obtain an ordinary CKY tensor. We also show that thanks to the GCKY tensors, the Maxwell equation reduces to three Debye equations, which are scalar-type equations, and two of them can be solved by separation of variables.
I. INTRODUCTIONHidden symmetry of spacetime has played an important role in the study of black hole physics. In particular, conformal Killing-Yano symmetry, known as hidden symmetry of the Kerr spacetime, has received special attention since it has been crucial to understanding the separability of various equations on a curved spacetime.On the Kerr spacetime, the Hamilton-Jacobi equation for geodesics, the Klein-Gordon equation, and the Dirac equation can be solved by separation of variables, and these separabilities have been understood in terms of a Killing-Yano tensor. For electromagnetic and gravitational perturbations, Teukolsky [1, 2] provided the scalar-type master equations, which can be solved by separation of variables. Cohen and Kegeles [3] showed that, if a spacetime is algebraically special and its repeated principal null direction (PND) is shear-free and geodesic, 1 the Maxwell equation reduces to a scalar-type master equation called the Debye equation. They also showed that when their method is applied to the Kerr spacetime, the Debye equation coincides with the Teukolsky equation. Moreover, they extended their results to massless Dirac fields on algebraically special spacetimes and gravitational perturbations on vacuum spacetimes [4]. After a while, Benn, Charlton, and Kress [5] unveiled the underlying structure of the works by Cohen and Kegeles from the viewpoint of GCKY symmetry. So far, while the GCKY symmetry has been related to obtaining the scalar-type master equations from the Dirac, Maxwell, and linearized Einstein equations, the separability of those equations has not been well understood. To fill the gap is one of the aims in this paper.In this paper, we examine hidden symmetry of the Wahlquist spacetime [6][7][8][9][10][11][12], which is a stationary, axially symmetric solution to the Einstein equation with rigidly rotating perfect fluids. According to the Goldberg-Sachs theorem, the repeated PNDs on a type-D vacuum spacetime are shear-free and geodesic. However, since the Wahlquist metric is a non-vacuum type-D spacetime, it is interesting to ask if the repeated PNDs are shear-free and geodesic. According to [3,5], if they are so, we obtain GCKY tensors with a certain condition and hence the Maxwell equation reduces to the Debye equation. Since the Wahlquist spacetime is known to admit a torsional conformal Killing-Yano (TCKY) tensor [12], there may...