Spectral evidence for a powerful compact jet from XTE J1118+480

We present observations of the X-ray transient XTE J1118+480 during its low /hard X-ray state outburst in 2008, ed radio and submillimetre wavelengths with the VLA, Ryle Telescope, MERLIN and JCMT. The high-resolution MERLIN o bservations reveal all the radio emission (at 5 GHz) to come from a compact core with physical dimensions smaller than 65d (kpc) au. The combined radi o data reveal a persistent and inverted radio spectrum, with spectral index similar to +0.5, The source is also detected at 350 GHz, on an extrapolati on of the radio spectrum. Flat or inverted radio spectra are now known to b e typical of the low/ hard X-ray state, and are believed to arise in synchr otron emission from a partially self-absorbed jet. Comparison of the radio and submillimetre data with reported near-infrared observations suggest tha t the synchrotron emission from the jet extends to the near-infrared, or po ssibly 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 argu ments we conclude that during the period of our observations XTE J1118+480 was producing a powerful outflow which extracted a large fraction of the to tal accretion power.