The timing properties of Sco X-1 along its Z track with EXOSAT

We present a systematic, homogeneous analysis of all the EXOSAT high time resolution data on Sco X-1. We investigate, for the first time, all power spectral properties of the under 100 Hz quasi-periodic oscillations (QPO) and noise of Sco X-1 as a function of position on the Z-shaped track traced out in the X-ray color-color diagram. We introduce a generally applicable method for parametrizing the position of a source on its track in a color-color or hardness-intensity diagram. Generally, the properties of Sco X-1 vary smoothly as a function of position along the Z track. However, some variability parameters change abruptly at either of the vertices of the Z track, indicating that the branches of the Z track represent distinct source states not only in spectral state but also in rapid variability characteristics. All variability components are found to have energy spectra harder than the average flux. We show that the very low-frequency noise is consistent with being solely the result of motion along the Z track. We study the high-frequency noise component for the first time in sufficient detail to show that there are changes in its cut-off frequency with position on the Z track. We find several indications that position on the Z track is not the only parameter governing the behavior of Sco X-1. (Author)