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Published September 1999 | Published
Journal Article Open

Quasi-Periodic Oscillations in Black Hole Candidates as an Indicator of Transition Between Low and High States

Abstract

By comparing positions on a spectral color-color diagram from 10 black hole candidates (BHCs) observed with Ginga (1354-64, 1826-24, 1630-47, LMC X-1, LMC X-3, GS 2000+25, GS 2023+33, GS 1124-68, Cyg X-1, and GX 339-4) with the observed broadband noise (BBN) (0.001-64 Hz) and quasi-periodic oscillation (QPO) variability, we find that the "very high state" is spectrally intermediate to the soft/high state and hard/low state. We find a transition point in spectral hardness where the dependence of the BHC QPO centroid frequency (of GS 1124-68 and GX 339-4) on spectral hardness switches from a correlation to an anticorrelation; where the BBN variability switches from high state to low state; and where the spectral hardness of the QPO relative to that of the BBN variability is a maximum. This coincidence of changing behavior in both the QPO and the broadband variability leads us to hypothesize that the QPO is due to interaction between the physical components which dominate the behaviors of BHCs when they occupy the hard/low and soft/high states. We conclude that these QPOs should be observed from BHCs during transition between these two states. Comparison with QPO and BBN behavior observed during the 1996 transition of Cyg X-1 supports this hypothesis. We also report 1-3 Hz QPOs observed in GS 2000+25 and Cyg X-1 in the hard/low state, and we compare these to the QPOs observed in GS 1124-68 and GX 339-4.

Additional Information

© 1999. The American Astronomical Society. Received 1998 July 3; accepted 1999 February 23. R. R. is grateful to W. Cui for providing the unpublished spectral hardness ratios measured from Cyg X-1. R. R. is also grateful to L. Bildsten for his hospitality at UC Berkeley where this work was completed. T. O. acknowledges an ESA Research Fellowship. M. K. gratefully acknowledges the Visiting Miller Professor Program of the Miller Institute for Basic Research in Science (UCB). W. H. G. L. acknowledges support from NASA.

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