Mixing of spherical and spheroidal modes in perturbed Kerr black holes

Abstract

The angular dependence of the gravitational radiation emitted in compact binary mergers and gravitational collapse is usually separated using spin-weighted spherical harmonics ${}_sY_{\ell m}$ of spin weight $s$, that reduce to the ordinary spherical harmonics $Y_{\ell m}$ when $s=0$. Teukolsky first showed that the perturbations of the Kerr black hole that may be produced as a result of these events are separable in terms of a different set of angular functions: the spin-weighted spheroidal harmonics ${}_sS_{\ell m n}$, where $n$ denotes the "overtone index" of the corresponding Kerr quasinormal mode frequency $\omega_{\ell m n}$. In this paper we compute the complex-valued scalar products of the ${}_sS_{\ell m n}$'s with the ${}_sY_{\ell m}$'s ("spherical-spheroidal mixing coefficients") and with themselves ("spheroidal-spheroidal mixing coefficients") as functions of the dimensionless Kerr parameter $j$. Tables of these coefficients and analytical fits of their dependence on $j$ are available online for use in gravitational-wave source modeling and in other applications of black-hole perturbation theory.