We present observations of the X-ray transient XTE J1118+480 during its Low/Hard X-ray state outburst in 2000 , at radio and sub-millimetre wavelengths with the VLA , Ryle Telescope , MERLIN and JCMT .
The high-resolution MERLIN observations reveal all the radio emission ( at 5 GHz ) to come from a compact core with physical dimensions smaller than 65 d ( kpc ) AU .
The combined radio data reveal a persistent and inverted radio spectrum , with spectral index \sim + 0.5 .
The source is also detected at 350 GHz , on an extrapolation of the radio spectrum .
Flat or inverted radio spectra are now known to be typical of the Low/Hard X-ray state , and are believed to arise in synchrotron emission from a partially self-absorbed jet .
Comparison of the radio and sub-millimetre data with reported near-infrared observations suggest that the synchrotron emission from the jet extends to the near-infrared , or possibly even optical regimes .
In this case the ratio of jet power to total X-ray luminosity is likely to be P _ { J } / L _ { X } > > 0.01 , depending on the radiative efficiency and relativistic Doppler factor of the jet .
Based on these arguments we conclude that during the period of our observations XTE J1118+480 was producing a powerful outflow which extracted a large fraction of the total accretion power .