Four persistent ( Cygnus X-1 , GX 339-4 , GRS 1758-258 and 1E 1740.7-2942 ) and three transient ( GS 2023+38 , GRO J0422+32 and GS 1354-64 ) black hole X-ray binary systems have been extensively observed at radio wavelengths during extended periods in the Low/Hard X-ray state , which is characterised in X-rays by a hard power-law spectrum and strong variability .
All seven systems show a persistent flat or inverted ( in the sense that \alpha \ga 0 , where S _ { \nu } \propto \nu ^ { \alpha } ) radio spectrum in this state , markedly different from the optically thin radio spectra exhibited by most X-ray transients within days of outburst .
Furthermore , in none of the systems is a high-frequency cut-off to this spectral component detected , and there is evidence that it extends to near-infrared or optical regimes .
Luminous persistent hard X-ray states in the black hole system GRS 1915+105 produce a comparable spectrum .
This spectral component is considered to arise in synchrotron emission from a conical , partially self-absorbed jet , of the same genre as those originally considered for Active Galactic Nuclei .
Whatever the physical origin of the Low/Hard X-ray states , these self-similar outflows are an ever-present feature .
The power in the jet component is likely to be a significant ( \geq 5 % ) and approximately fixed fraction of the total accretion luminosity .
The correlation between hard X-ray and synchrotron emission in all the sources implies that the jets are intimately related to the Comptonisation process , and do not have very large bulk Lorentz factors , unless the hard X-ray emission is also beamed by the same factor .