We report multiwavelength observations of the soft X-ray transient ( SXT ) XTE J1118+480 , which we observed with UKIRT , HST , RXTE , EUVE and many other instruments and facilities .
Adding radio ( Ryle Telescope , VLA ) , sub-millimeter ( JCMT ) and X-ray ( Chandra and SAX ) data from the literature , we assembled the most complete spectral energy distribution ( SED ) of this source yet published .
We followed the evolution of this source for 1 year , including 6 observations performed during the outburst , and one observation at the end of the outburst .
Because of XTE J1118+480 ’ s unusually high galactic latitude , it suffers from very low extinction , and its SED is near-complete , including EUV ( Extreme Ultraviolet ) observations . XTE J1118+480 exhibits an unusually low low-hard state ( estimated inner radius 350 R _ { s } ) and a strong non-thermal contribution in the radio to optical domain , which is likely to be due to synchrotron emission .
We discuss the interstellar column density and show that it is low , between 0.80 and 1.30 \times 10 ^ { 20 } \mbox { cm } ^ { -2 } .
We analyse the evolution of the SED during the outburst , including the contributions from the companion star , the accretion disc , the outflow , and relating irradiation and variability of the source in different bands to the SED .
We find no significant spectral variability during the outburst evolution , consistent with the presence of a steady outflow .
Analysis of its outflow to accretion energy ratio suggests that the microquasar XTE J1118+480 is analogous to radio-quiet quasars .
This , combined with the inverted spectrum from radio to optical , makes XTE J1118+480 very similar to other microquasar sources , e.g . GRS 1915+105 and GX 339-4 in their low/hard state .
We model the high-energy emission with a hot disc model , and discuss different accretion models for XTE J1118+480 ’ s broad band spectrum .