Autocorrelation function. The ACFs are shown as the grey curves with a smoothed version in black. The (coloured) periodogram is calculated from the smoothed ACF. The y-axis on the left (right) is for the ACF (periodogram). Top: the 2 min cadence ACF with the corresponding periodogram. The vertical grey line denotes the measured rotation period, and the dashed lines are integer values of this value. Shown as the shaded area is the confidence interval for the rotation period. Bottom: the 30 min cadence ACF.