Cross-correlation of the pulse profiles from NICER and Aqueye+ observations of February 2019 as the lag varies. The top and middle panels show NICER and Aqueye+ pulse profiles with 64 phase bins, respectively. Solid lines indicate the best-fitting two-harmonic function. The bottom panel shows the cross-correlation coefficient as a function of the lag with which the optical pulse profile is shifted. We used phase lags ranging from −32 to +32, including the zero lag (dashed vertical line).