we propose a jet model for the low/hard state of black-hole X-ray sources which explains a) the X-ray spectra, b) the timelag spectra, c) the increase in the amplitude (QPO and high frequency) with increasing photon energy, and d) the narrowing of the autocorrelation function with increasing photon energy. The model (in its simplest form) assumes that i) there is a uniform magnetic field along the axis of the jet, ii) the electron density in the jet is inversely proportional to distance and iii) the jet is hotter near its center than at its periphery. We have performed Monte Carlo simulations of Compton upscattering of soft photons from the accretion disk and have found power-law high-energy spectra with photon number index in the range 1.5-2, power-law timelags versus Fourier frequency with index ~0.8, and an increase of the rms amplitude of the variability and a narrowing of the autocorrelation function with photon energy as they have been observed in Cygnus X-1.
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