Finite-Window Centered Organization of Neighboring Poles

Abstract

Near-degenerate resonance poles arise widely in open-wave systems. For gravitational-wave ringdowns, inference is performed on finite time windows where neighboring quasinormal modes can be spectrally close; the waveform is then dominated by a common carrier with a slowly varying interference envelope, while representing the signal as a sum of two independently resolved damped exponentials $e^{-\iiω_\pm t}$ becomes numerically ill-conditioned when the dimensionless splitting $η=|σ|T_{\mathrm{eff}}$ is small. We give a finite-window organizing principle for such neighboring-pole sectors: the local two-pole singular block of the Green-function integrand is rewritten exactly about a shared carrier $ω_c$ and half-splitting $σ$, and for $|σt|\ll 1$ the time-domain projection is systematically a carrier plus a first-jet piece $\propto t\,e^{-\iiω_c t}$, without requiring a literal double pole or exceptional-point merger in parameter space. The centered first-jet basis has $O(1)$ Gram conditioning, whereas the resolved-mode basis satisfies $\mathrm{cond}(G_{\mathrm{res}})\sim 12\,η^{-2}$ as $η\to 0$ (transparent real-splitting slice). We supply finite-window diagnostics in which $κ$ marks when the jet correction must be retained and $η^2$ sets the residual error scale once it is retained. Minimal two-pole numerics verify the scaling. For Kerr black holes we fix one adjacent-overtone mode pair (catalog label \texttt{pair45}; shared $(l,m)$ and consecutive overtones in our indexed tabulation), scan spin $a\in[0.8770,0.8810]$, and adopt the spectral window proxy $T_{\mathrm{spec}}=β/|\Imω_c|$ with $β=2.0$ to illustrate the same conditioning contrast in a near-degenerate sector.