Time lags relative to the second harmonic term (Table 5). The y-axis is not temporally equispaced. The purple point indicates the values for simultaneous observations between XMM-Newton and SiFAP2, blue points show values between XMM-Newton and Aqueye+, light blue points indicate values between NICER and SiFAP2, and green points show the values between NICER and Aqueye+. The dashed line indicates a zero time lag. Colored error bars represent 1σ statistical uncertainties, while the black error bars indicate the total error. The influence of the absolute timing accuracy of XMM-Newton (∼100 μs) and SiFAP2 (∼60 μs) is visible.