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

We present a systematic, homogeneous analysis of all the EXOSAT ME, high ti me resolution data on Sco X-1. We investigate, for the first time, all powe r spectral properties of the < 100 Hz quasi-periodic oscillations (QPO) and noise of Sco X-1 as a function of position on the Z-shaped track traced ou t in the X-ray colour-colour diagram. For this purpose, we introduce a new generally applicable method for parametrizing the position of a source on i ts track in a colour-colour or hardness-intensity diagram. Generally, the p roperties of Sco X-1 vary smoothly as a function of position along the Z tr ack. However, some variability parameters change abruptly at either of the vertices of the Z track, indicating that the branches of the Z track repres ent distinct source states not only in spectral state but also in rapid var iability characteristics. All variability components are found to have ener gy spectra harder than the average flux. We show that the very low-frequenc y noise (VLFN) is consistent with being solely the result of motion along t he Z track. The power spectra of the X-ray intensity as well as source posi tion along the Z track extend, unbroken, to time-scales of nearly 1 day. We study the high-frequency noise (HFN) component for the first time in suffi cient detail to show that there are changes in the HFN cut-off frequency wi th position on the Z track. It changes abruptly from similar to 75 to simil ar to 35 Hz at the normal/flaring branch vertex. The HFN is found to extend out to about 300 Hz. The QPO show a remarkably rapid change in frequency a t or just before the normal-branch/flaring-branch (NB/FB) vertex. This tran sition happens within 1.5 per cent of the entire extent of the Z track. The QPO themselves are visible for 17 per cent of the Z. We find a new type of behaviour near the NB/FB vertex, i.e., rapid excursions from the NB into t he FB and back again taking only a few minutes. We find several indications that position on the Z track is not the only parameter governing the behav iour of Sco X-1. The most dramatic examples of this are two brief episodes where the QPO frequency changed rapidly without the usual changes in colour s and intensity that accompany a change of source state. In one case the fr equency rapidly rose from 6 to 16 Hz, and then returned to 6 Hz, while Sco X-1 apparently remained unmoved on the normal branch in the colour-colour d iagram. In the second case the QPO frequency changed from 16 to 7 Hz withou t the usual indications of a passage through the normal/flaring branch vert ex (simultaneous dips in the count rate and colours as the frequency change s through 8 Hz). Thus it seems that deviations from the usual one-to-one co rrespondence between QPO behaviour and spectral state, perhaps caused QPO f requency mode switching, do occasionally occur.