Extended parameterized spin expansion formalism for ringdown analysis with GW250114

Abstract

Parameterized descriptions of black-hole quasinormal-mode spectra are essential for testing gravity with ringdown observations. We extend the Parameterized Spin Expansion Coefficients (ParSpec) formalism by simultaneously sampling the characteristic length scale $\tilde{\ell}$ and the scaling index $\tilde{p}$, rather than fixing $\tilde{p}$ to a theory-motivated integer and constraining $\ell$. Physically, this extension promotes the scaling of the spectral corrections coming from higher-curvature operators to an observable quantity. Methodologically, it enables us to investigate the enlarged ParSpec parameter space and identify the prior geometry induced by conditions on the effective coupling $\gamma$. We examine the robustness of the framework by using the $220$ and $220+221$ ringdown models over different start times with informative priors on mass and luminosity for GW250114, and further study a joint constraint with GW231123. We find that $\tilde{p}$ remains prior dominated and that the data show no evidence for deviations from general relativity (GR). Among the coupling prescriptions considered, $\gamma<1$ avoids an artificial correlation between $\tilde{p}$ and $\tilde{\ell}$. At the current signal-to-noise ratio, the results based on the Kullback-Leibler divergence show that the $220$-only model provides more informative constraints than the $220+221$ model. Higher-SNR ringdowns and hierarchical analyses of a larger event population will be required to break the $\tilde{p}-\tilde{\ell}$ degeneracy and directly probe the scaling structure of corrections to GR.