  Most, probably all, accreting binaries that are believed to contain a
black-hole emit radio waves when they are in the low/hard state. Whenever this
radio emission has been resolved, a jet-like structure has become apparent. We
propose that Compton upscattering of low-energy photons in the jet can explain
both the energy spectra and the time lags versus Fourier frequency observed in
the low/hard state of black-hole systems. The soft photons originate in the
inner part of the accretion disk. We have performed Monte Carlo simulations of
Compton upscattering in a jet and have found that for a rather wide range of
values of the parameters we can obtain power-law high-energy X-ray spectra with
photon-number index in the range 1.5 - 2 and power-law time lags versus Fourier
frequency with index ~ 0.7. The black-hole source Cyg X-1 in the low/hard state
is well described by our model.
