2023MNRAS.526..645I

We observed the low-mass X-ray binary Sco X-1 for 12 nights simultaneously using the Rossi X-Ray Timing Explorer and the Otto Struve Telescope at McDonald Observatory at 1-s time-resolution. This is among the most comprehensive simultaneous X-Ray/optical data sets of Sco X-1. Evidence of reprocessing was observed in the form of nine positive, near-zero lag peaks in the cross-correlation function, eight of which were relatively small, and took the shape of piecewise exponential functions. These peaks were initially identified by eye, after which a computational identification scheme was developed to confirm their significance. Based on their short lags (less than 4 s), as well as their occurrence on the flaring branch (FB) and soft apex, the small cross-correlation features are likely to be caused by reprocessing off the outer disc, although the companion could still make a contribution to their tails. The Z track was parameterized using a rank number scheme so that the system's location on the track could be numerically defined. Plotting the results against the optical reveals an increasing step function when moving from the horizontal branch to the normal branch (NB) to the FB, with differential optical levels at ∼0.47, ∼0.57, and ∼1.1, respectively. An additional correlation between Z track location and the optical was found on the upper FB. An optical intensity histogram reveals a transition region between the NB and FB with only intermediate fluxes.