Generalized Lomb-Scargle periodograms of several activity indicators of GJ 251 from CARMENES spectroscopic data. The dashed black periodograms represent the GLS of the activity indicators, and the solid GLS periodogram represents the residuals after subtracting a 365 d signal. For the residuals from which the 365 d signal was subtracted, we also overplot the s-BGLS periodogram, where the probability increases from blue to white to red. The red dashed lines mark the rotation period and the first harmonic estimated from photometric data, while the dotted red lines show the 3σ uncertainties. The dashed black line marks a significant HIRES signal around 600 d, and the yearly period of 365 d. The dashed yellow line marks the period of the planetary signal published in this work.