2002MNRAS.336..217O

Sco X-2 (GX 349+2) is a low-mass X-ray binary and Z source. We have analysed ∼156 ks of Rossi X-ray Timing Explorer data, obtained in 1998 January, on this source. We investigated the fast-time variability as a function of position on the Z track. During these observations, Sco X-2 traced out the most extensive Z track ever reported from this object, making this the most comprehensive study thus far. We found the broad peaked flaring branch noise that is typical of Sco X-2, with a centroid frequency in the range 3.3-5.8 Hz. We also discovered low-frequency noise, and a new peaked noise feature, with centroid frequencies in the range 5.4-7.6 and 11-54 Hz, respectively. We discuss the phenomenology of these features, their relationship with the power spectral components found in other low-mass X-ray binaries, and the implications for current models. In particular, the low-frequency noise we observed was strongest at intermediate energies, in contrast to the low-frequency noise seen in other Z sources. We also detected very low-frequency noise, and have calculated complex cross-spectra between intensity and hardness. We found that the very low-frequency noise is not entirely a result of motion along the Z track.