Astronomy and Astrophysics – Astrophysics
Scientific paper
1999-09-28
Astronomy and Astrophysics
Astrophysics
30 pages, 15 figures, MNRAS in press
Scientific paper
10.1046/j.1365-8711.2000.03050.x
We present a systematic, homogeneous analysis of all the EXOSAT ME, high time resolution data on Sco X-1. We investigate all power 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 out in the X-ray colour-colour 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. We show that the very low frequency noise (VLFN) is consistent with being solely due to motion along the Z track. The QPO show a remarkably rapid change in frequency at or just before the normal-branch/flaring-branch (NB/FB) vertex. This transition happens within 1.5% of the entire extent of the Z track. The QPO themselves are visible for 17% of the Z. We find a new sort of behaviour near the NB/FB vertex, i.e., rapid excursions from the NB into the FB and back again taking only a few minutes.
Dieters Stefan W.
van der Klis Michiel
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