Quasinormal-mode residues and pole coalescence in hypergeometric black-hole models

Abstract

We study source-normalized quasinormal-mode residues in radial boundary-value problems reducible to the Gauss hypergeometric equation. Hypergeometric connection coefficients separate the spectral condition from the normalization and spatial dependence of the frequency-domain Green function. We illustrate this construction for Dirichlet modes of the rotating Bañados--Teitelboim--Zanelli black hole and mixed boundary conditions in two-dimensional anti-de Sitter spacetime, and then apply it to the Pöschl--Teller/Nariai problem. Near pole coalescence, we obtain finite-position residues for the two simple poles, while at the critical point both coefficients of the second-order Laurent expansion are reduced to closed finite sums. The subleading coefficient depends explicitly on the frequency derivative of the Green-function numerator and is therefore not determined by the spectral function alone. We also identify the range in which the usual vanishing-discriminant criterion characterizes the Pöschl--Teller coalescence: it holds for the positive barrier considered here, whereas other double zeros or cancellations can occur outside that regime. These results provide a normalization-controlled analytic benchmark for relating spectral coalescence to sourced quasinormal response.