EDP Sciences
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Volume 412, Number 1, December II 2003
Page(s) 235 - 240
Section Stellar atmospheres
DOI http://dx.doi.org/10.1051/0004-6361:20031423

A&A 412, 235-240 (2003)
DOI: 10.1051/0004-6361:20031423

A transient variable 6 Hz QPO from GX 339-4

E. Nespoli1, 2, T. Belloni1, J. Homan1, J. M. Miller3, W. H. G Lewin4, M. Méndez5 and M. van der Klis6

1  INAF - Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy
2  Università degli Studi di Milano, via Celoria 16, 20133 Milano, Italy
3  Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
4  Center for Space Research, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139-4307, USA
5  SRON National Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
6  Astronomical Institute "Anton Pannekoek" University of Amsterdam and Center for High-Energy Astrophysics, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands

(Received 27 May 2003 / Accepted 8 September 2003)

We report the results of an observation with the Rossi X-ray Timing Explorer of the black hole candidate GX 339-4 during its 2002/2003 outburst. This observation took place during a spectral transition from the hard to the soft state. A strong (6% rms) transient quasi-periodic oscillation (QPO) appears suddenly in the power density spectrum during this observation. The QPO centroid is ~6 Hz, but it varies significantly between 5 and 7 Hz with a characteristic time scale of ~10 s, correlated with the 2-30 keV count rate. The appearance of the QPO is related to spectral hardening of the flux, due to a change in the relative contribution of the soft and hard spectral components. We compare this peculiar behavior with results from other systems that show similar low frequency QPO peaks, and discuss the results in terms of possible theoretical models for QPO production.

Key words: accretion: accretion disks -- black hole physics -- stars: oscillations -- X-rays: stars

Offprint request: T. Belloni, nespoli@merate.mi.astro.it

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