Infrared/X-ray cross-correlation functions for the first (left) and second (right) epoch, both calculated and plotted with a 62.5 ms time resolution. Positive lags mean that IR lag X-rays. A clear correlation is detected in the first epoch, with a peak at ≈ − 0.13 s. The CCF of the second epoch shows a complex structure, with multiple anti-correlation and correlation features. As shown by the sinusoidal wave (with a period of 4 s), most of the peaks seem to be associated with the QPO at 0.25 Hz visible in the IR PDS (Fig. 3)