  We propose a jet model for the low/hard state of Galactic black-hole X-ray
sources that can explain the energy spectra from radio to X-rays. The model
assumes that i) there is a magnetic field along the axis of the jet; ii) the
electron density in the jet drops inversely proportional to distance; and iii)
the electrons in the jet follow a power law distribution function. 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 and cutoff at a few hundred keV. The
spectrum at long wavelengths (radio, infrared, optical) is modeled to come from
synchrotron radiation of the energetic electrons in the jet. We find flat to
inverted radio spectra that extend from the radio up to about the optical band.
For magnetic field strengths of the order of 10^5-10^6G at the base of the jet,
the calculated spectra agree well in slope and flux with the observations. Our
model has the advantage over other existing models that it also explains many
of the existing timing data such as the time lag spectra, the hardening of the
power density spectra and the narrowing of the autocorrelation function with
increasing photon energy.
