1997A&A...322..857B

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 9-30keV source count-rate. We found a previously unknown 6.7Hz 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.