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Astron. Astrophys. 322, 857-867 (1997)


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Energy dependence in the quasi-periodic oscillations and noise of black hole candidates in the very high state

T. Belloni 1, M. van der Klis 1, W.H.G. Lewin 2, J. van Paradijs 1, 3, T. Dotani 4, K. Mitsuda 4 and S. Miyamoto 5

1 Astronomical Institute "Anton Pannekoek", University of Amsterdam and Center for High-Energy Astrophysics, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
2 Massachusetts Institute of Technology, Center for Space Research, Room 37-627, Cambridge, MA 02139, USA
3 Physics Department, University of Alabama in Huntsville, AL 35899, USA
4 Institute of Space and Astronautical Science, 3-1-1, Yoshinodai, Sagamihara, Kanagawa 229, Tokyo, Japan
5 Osaka University of Health and Sport Science, Noda 1558-1, Kumatori-cho, Sennan-gun, Osaka, 590-04, Japan

Received 28 May 1996 / Accepted 10 December 1996

Abstract

We present the results of an energy-dependent timing analysis of Ginga data from the black hole candidates GS 1124-68 and GX 339-4 in their Very High State (VHS). Large variations with energy of the timing properties are seen in both sources. The break frequency of the band-limited noise increases with energy, indicating considerable spectral changes on sub-second timescales. Quasi-periodic oscillations (QPOs) with high (and energy dependent) harmonic content are observed, whose peaks in the power spectra appear to be asymmetric. The fractional amplitude of the fundamental QPO peak increases with energy and the QPO harmonic content decreases with energy, indicating that the at higher energies the QPO cycle becomes more sinusoidal. During one observation of GS 1124-68, the QPO centroid frequency increases with decreasing 9-30 keV source count-rate. We found a previously unknown 6.7 Hz QPO peak, also energy dependent, in GS 1124-68, four months after the peak of the outburst. The energy dependence of the band-limited noise is interpreted in the frame of a shot-noise model with softening shots. The complex phenomenology reported here puts strong constraints on the theoretical models for spectral distribution and time variability of black hole candidates at high accretion rates.

Key words: X-rays: stars – stars: individual, GS 1124-68, GX 339-4 – binaries: close

Send offprint requests to: T. Belloni

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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