Background-subtracted rms amplitudes of X-ray (top panel) and optical (middle panel) pulse during NICER/Aqueye+ simultaneous observations in February 2019, January 2020, and January–February 2022. The arrows represent the upper limits converted into rms amplitudes at 3σ confidence level computed when the optical pulsations were not detected. In the lower panel, we show the phases of the optical pulse (red dots and empty yellow squares show the first and second harmonic, respectively), and the phases of the X-ray pulse (blue dots and light blue empty squares show the first and second harmonic, respectively) during these simultaneous observations.