  Some recent observational results impose significant constraints on all the
models that have been proposed to explain the Galactic black-hole X-ray sources
in the hard state. In particular, it has been found that during the hard state
of Cyg X-1 the power-law photon number spectral index is correlated with the
average time lag between hard and soft X-rays. Furthermore, the peak
frequencies of the four Lorentzians that fit the observed power spectra are
correlated with both the photon index and the time lag. We performed Monte
Carlo simulations of Compton upscattering of soft, accretion-disk photons in
the jet and computed the time lag between hard and soft photons and the
power-law index of the resulting photon number spectra. We demonstrate that our
jet model naturally explains the above correlations, with no additional
requirements and no additional parameters.
