GJ 581 Hα. Top panel: the GLS periodogram ﹩{\hat{S}}^{LS}({f}_{k})﹩ (green) with the best-fit AR(1) (purple solid line), power law (red dashed line), and white noise (black dashed–dotted line) models. Navy dotted lines mark the rotation signal and its first and second harmonics. Bottom panel: the red noise 5% (red), 1% (orange), and 0.1% (blue) FALs and the 1% white noise FAL (black dashed line) overlaid on the GLS periodogram. The rotation signal reaches a 99.9% significance level according to both the red and white noise FALs.