Rotating black hole orbit functionals in the frequency domain

In many astrophysical problems, it is important to understand the behavior of functions that come from rotating (Kerr) black hole orbits. It can be particularly useful to work with the frequency domain representation of those functions, in order to bring out their harmonic dependence upon the fundamental orbital frequencies of Kerr black holes. Although, as has recently been shown by Schmidt, such a frequency domain representation must exist, the coupled nature of a black hole orbit’s r and θ motions makes it difficult to construct such a representation in practice. Combining Schmidt’s description with a clever choice of timelike coordinate suggested by Mino, we have developed a simple procedure that sidesteps this difficulty. One first Fourier expands all quantities using Mino’s time parameter λ. In particular, the observer’s time t is decomposed with λ. The frequency domain description is then built from the λ-Fourier expansion and the expansion of t. We have found this procedure to be quite simple to implement, and to be applicable to a wide class of functionals. We test the procedure using a simple test function, and then apply it to a particularly interesting case, the Weyl curvature scalar ψ₄ used in black hole perturbation theory.