  Galactic black-hole X-ray binaries emit a compact, optically thick, mildy
relativistic radio jet when they are in the hard and hard-intermediate states.
In a series of papers, we have developed a jet model and have shown, through
Monte Carlo simulations, that our model can explain many observational results.
In this work, we investigate one more constraining relationship between the
cutoff energy and the phase lag during the early stages of an X-ray outburst of
the black-hole X-ray binary GX 339-4: the cutoff energy decreases while the
phase lag increases during the brightening of the hard state. We demonstrate
that our jet model naturally explains the above correlation, with a minor
modification consisting of introducing an acceleration zone at the base of the
jet. The observed correlation between the cutoff energy and the phase lag
suggests that the lags are produced by the hard component. Here we show that
this correlation arises naturally if Comptonization in the jet produces these
two quantities.
