A pre-merger-informed spectral-level ringdown inference framework for black-hole spectroscopy

Black-hole spectroscopy aims to infer properties of the remnant spacetime from the quasinormal-mode (QNM) spectrum of the gravitational-wave ringdown signal. In most implementations, however, this inference is performed with waveform models that already incorporate Kerr or other theory-specific QNM spectral relations, thereby entangling spectral measurement with remnant or beyond-Kerr parameter inference. At the same time, conventional ringdown analyses commonly excise the pre-merger data, which in principle contain information about the excitation amplitudes and phases of the QNMs. We introduce \texttt{SPRING} (\textit{Spectral-level Pre-merger-informed RINGdown inference}), a framework designed to separate ringdown spectral inference from theory-side interpretation while propagating pre-merger information through amplitude-scale estimation. As a demonstration, we apply \texttt{SPRING} to GW250114 using an agnostic two-component damped-sinusoid (2DS) model for Kerr remnant inference. \texttt{SPRING} improves the Bayesian support for the agnostic 2DS signal model relative to analyses that do not use pre-merger information, with an increase of $\Delta\ln B\sim 5$--$10$. The resulting remnant posterior remains closely consistent with the inspiral-merger-ringdown estimate, despite the extra freedom introduced by the second DS component. This work bridges pre-merger information and ringdown inference, establishing a fully spectral-level route for future black-hole spectroscopy.