  Most black hole candidate X-ray binaries show Fourier time lags between
softer and harder X-rays. The hard photons seem to arrive up to a few ms after
the soft for a given Fourier frequency of the perturbation. The energy
dependence of the time lags has a roughly logarithmic behavior. Up to now most
theories fail to explain the observed magnitude and Fourier frequency
dependence of the lags or fail other statistical tests. We show that the time
lags can arise from a simple pivoting power law model, which creates the
logarithmic dependence on the photon energy at once. A pivoting power law
arises naturally from jet/synchrotron models for the X-ray emission, but may
also be applicable to corona models. A hint to the coherence features of the
light-curves can be obtained from the power spectral density, which can be
decomposed into a few broad Lorentzians that could arise from a couple of
strongly damped oscillators with low quality factors below one. Using small
variations of the power law index for each Lorentzian separately the lags can
be derived analytically. They show the correct Fourier frequency dependence of
the time lags. If one assumes variations of the power law index by +- 0.2 the
model can account for the observed magnitude of the time lags in Cyg X-1. As a
further test we calculated the cross- and auto-correlation functions for our
model, which also show qualitatively the observed behavior. The coherence
function for the model is in agreement with the observed data in the Fourier
regime, where the model is valid.
