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Astron. Astrophys. 332, 845-848 (1998)

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

Erik Kuulkers 1 and Michiel van der Klis 2

1 Astrophysics, University of Oxford, Nuclear and Astrophysics Laboratory, Keble Road, Oxford OX1 3RH, UK (e-mail: erik@astro.ox.ac.uk)
2 Astronomical Institute "Anton Pannekoek", University of Amsterdam and Center for High-Energy Astrophysics, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands (e-mail: michiel@astro.uva.nl)

Received 4 November 1997 / Accepted 17 December 1997


We report on about 4 hrs of observations made with the Rossi X-ray Timing Explorer on 1997 May 2 of the low-mass 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 [FORMULA] 16 Hz) noise component peaking near 9 Hz, with a fractional rms amplitude of [FORMULA] 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 [FORMULA] 0.9% (2-30 keV) on any such QPO. In the FB the power spectrum showed a somewhat less broad noise component (FWHM [FORMULA] 11 Hz) peaking near 6 Hz with a fractional rms of [FORMULA] 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 hardness-intensity 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 [FORMULA] confidence) for a QPO near 1020 Hz with FWHM [FORMULA] 40 Hz and fractional rms of [FORMULA] 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.

Key words: accretion, accretion disks – binaries: close – stars: individual (GX 349+2, Sco X-2) – stars: neutron – X-rays: stars

Send offprint requests to: Erik Kuulkers

© European Southern Observatory (ESO) 1998

Online publication: March 30, 1998