GX 349+2 (Sco X-2): an odd-ball among the Z sources

We report on about 4 hrs of observations made with the Rossi X-ray Timing Explorer on 1997 May 2 of the lowmass X-ray binary and Z source GX 349+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∼16 Hz) noise component peaking near 9 Hz, with a fractional rms amplitude of ∼3% (2–30 keV). 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– 30 keV) on any such QPO. In the FB the power spectrum showed a somewhat less broad noise component (FWHM∼11 Hz) peaking near 6 Hz with a fractional rms of ∼4% (2–30 keV). 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 hardnessintensity diagram. By comparing GX 349+2 with the other Z and atoll sources, we conclude that GX 349+2 differs from the Z sources in various aspects and shows similarities to the behaviour seen in the bright atoll sources, such as GX 13+1 and GX 3+1. We also searched for kilo-Hertz QPO, similar to those present in other Z sources. We only found weak evidence (2.6 σ confidence) for a QPO near 1020 Hz with FWHM∼40 Hz 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 Sco X-1, GX 17+2 and Cyg X-2.