Extended parameterized spin expansion formalism for ringdown analysis with GW250114

Abstract

We extend the Parameterized Spin Expansion Coefficients (ParSpec) framework for ringdown tests of black holes by simultaneously sampling the characteristic length scale $\ell$ and the scaling index $p$, where the two parameters are replaced by $\tilde{p}$ and $\tilde{\ell}$ in subsequent analysis, respectively. Unlike previous analyses that fix $\tilde{p}$ to theory-motivated integer values, this extension provides a theory-agnostic description inspired by effective-field theory to the Kerr ringdown spectrum. Using \texttt{pyRing}, we apply the framework to GW250114 with informative priors on $M_{\rm f}$ and $D_{\rm L}$, comparing the $220$-only and $220+221$ mode models under different conditions on $γ$. The posterior distributions of $\tilde{\ell}, \tilde{p}$ and $γ$ are presented with different ringdown start times. We show that the posterior of $\tilde{p}$ remains largely prior dominated, indicating that current ringdown data cannot distinguish different scaling behaviors of the correction. The inferred constraint on $\tilde{\ell}$ is instead mainly controlled by the geometry of the allowed parameter space induced by the $γ$ condition. In particular, $γ<1$ provides the most natural prescription in the $(\tilde{\ell},\tilde{p})$ plane and yields stable but weak upper bounds of $\tilde{\ell}_{90}\simeq 83\, {\rm km}$. An analysis based on the Kullback-Leibler divergence further shows that, after accounting for the effective prior associated with $γ<1$, the data-driven information gain on $\tilde{\ell}$ and $\logγ$ is very small. At the current SNR level, the $220$-only model provides a more informative constraint than the $220+221$ model. We also perform a joint analysis including GW231123 and find that the combined constraint is dominated by GW250114.