The Impact of Spin Priors on Parameterized Tests of General Relativity

Abstract

Spin priors play a fundamental role in gravitational-wave parameter estimation, yet their impact on parameterized tests of General Relativity (GR) remains insufficiently understood. We systematically investigate how spin-prior choices affect the 1.5PN deviation parameter $\delta {\hat{\varphi }}_3$ using real gravitational-wave events. Prior-induced effects are quantified through the Jensen--Shannon divergence and posterior median shifts. We find that ${\chi }_p$ is significantly more sensitive to the spin prior than ${\chi }_{\mathrm{eff}}$, while $\delta {\hat{\varphi }}_3$ is generally robust but can exhibit larger shifts in low-information regimes. GW231123_135430 shows particularly strong sensitivity, but its inferred properties are subject to waveform-model and event-specific systematics, warranting caution in interpreting results strongly influenced by this event. When $\delta {\hat{\varphi }}_3$ is included in the parameter-estimation model, a strong correlation emerges between the prior sensitivities of ${\chi }_{\mathrm{eff}}$ and $\delta {\hat{\varphi }}_3$, consistent with their partial degeneracy at the 1.5PN order, whereas ${\chi }_p$ remains weakly correlated. A leave-one-out test shows that GW231123\_135430 significantly affects the correlation strength, but the correlation persists when this event is removed. These results demonstrate that spin-prior choices can propagate into parameterized tests of GR and should be carefully accounted for when interpreting apparent deviations from GR.

arXiv comment20 pages, 5 figures