Three States and Complexity–Pulse Intensity Decoupling in LIGO 55–65 Hz Noise

We analyze the 55–65 Hz noise band of LIGO H1, L1, and Virgo V1 using four independent nonlinear metrics (CV, envelope skewness, recurrence rate, sample entropy) across O1–O4. We find three distinguishable noise states rather than two, and observe that complexity (sample entropy) and pulse intensity (CV, envelope skewness) are decoupled: sample entropy saturates and then decreases, while CV and skewness increase monotonically. The bimodal feature is precisely localized to 55–60 and 60–65 Hz. Statistical validation gives 100% accuracy for O1 vs. O2/O3 and 94.9% for O2/O3 vs. O4. Data from GWOSC. Code available. License: CC BY 4.0.