Monodromy eigenvalues of the radial Teukolsky equation and their connection to the renormalized angular momentum

Abstract: The Teukolsky equation describes perturbations of Kerr spacetime and is central to the study of rotating black holes and gravitational waves. In the frequency domain, the Teukolsky equation separates into radial and angular ordinary differential equations. Mano, Suzuki, and Takasugi (MST) found semi-analytic solutions to the homogeneous radial Teukolsky equation in terms of series of analytic special functions. The MST expansions hinge on an auxiliary parameter known as the renormalized angular momentum $\nu$, which one must calculate to ensure the convergence of these series solutions. In this work, we present a method for calculating $\nu$ via monodromy eigenvalues, which capture the behavior of ordinary differential equations and their solutions in the complex domain near their singular points. We directly relate the monodromy data of the radial Teukolsky equation to the parameter $\nu$ and provide a numerical scheme for calculating $\nu$ based on monodromy. With this method we evaluate $\nu$ in different regions of parameter space and analyze the numerical stability of this approach. We also highlight how, through $\nu$, monodromy data are linked to scattering amplitudes for generic (linear) perturbations of Kerr spacetime.