Spectra and time variability of Galactic black-hole X-ray sources in the low/hard state

We propose a jet model for the low/hard state of galactic black-hole X-ray sources which explains a) the X-ray spectra; b) the time-lag spectra; c) the increase of the variability 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 time lags versus Fourier frequency with index similar to0.8, and an increase of the rms amplitude of variability and a narrowing of the autocorrelation function with photon energy as has been observed in Cygnus X-1. Similar energy spectra and time variability have been observed in neutron-star systems in the island state. It is therefore quite likely that a similar model holds for these sources as well.