# Rotating black hole orbit functionals in the frequency domain

## TL;DR

This paper introduces a new method to construct frequency domain representations of functions related to Kerr black hole orbits by combining Mino's coordinate with Fourier expansion, simplifying analysis in astrophysics.

## Contribution

The authors develop a straightforward procedure to obtain frequency domain functionals for Kerr black hole orbits, overcoming previous practical difficulties by combining Schmidt's and Mino's approaches.

## Key findings

- The method is simple to implement and broadly applicable.
- Validated with a test function and applied to Weyl curvature scalar .
- Facilitates analysis of black hole perturbations in the frequency domain.

## Abstract

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 W. Schmidt, such a frequency domain representation must exist, the coupled nature of a black hole orbit's $r$ and $\theta$ motions makes it difficult to construct such a representation in practice. Combining Schmidt's description with a clever choice of timelike coordinate suggested by Y. Mino, we have developed a simple procedure that sidesteps this difficulty. One first Fourier expands all quantities using Mino's time coordinate $\lambda$. In particular, the observer's time $t$ is decomposed with $\lambda$. The frequency domain description is then built from the $\lambda$-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 in a particularly interesting case, the Weyl curvature scalar $\psi_4$ used in black hole perturbation theory.