1998A&A...332..845K

We report on about 4hrs of observations made with the Rossi X-ray Timing Explorer on 1997 May 2 of the low-mass X-ray binary and Z source GX349+2 (Sco X-2). Initially the source was in the normal branch (NB), later it moved to the flaring branch (FB). In the NB the power spectra reveal a broad (FWHM∼16Hz) noise component peaking near 9Hz, with a fractional rms amplitude of ∼3% (2-30keV). This noise component does not resemble the strong quasi-periodic oscillations (QPO) usually seen in the NB of other Z sources. We set 95% confidence upper limits on the fractional rms of ∼0.9% (2-30keV) on any such QPO. In the FB the power spectrum showed a somewhat less broad noise component (FWHM∼11Hz) peaking near 6Hz with a fractional rms of ∼4% (2-30keV). We compare our results with previous reports, and find that the fast timing behaviour changes not only with position in the Z, but also as a function of the position of the Z track in the hardness-intensity diagram. By comparing GX349+2 with the other Z and atoll sources, we conclude that GX349+2 differs from the Z sources in various aspects and shows similarities to the behaviour seen in the bright atoll sources, such as GX13+1 and GX3+1. We also searched for kilo-Hertz QPO, similar to those present in other Z sources. We only found weak evidence (2.6σ confidence) for a QPO near 1020Hz with FWHM∼40Hz and fractional rms of ∼1%. We note that this frequency and fractional rms are consistent with those expected and observed in the lower NB/FB of the Z sources ScoX-1, GX17+2 and CygX-2.