A measurement of the broadband spectrum of XTE J1118+480 with BeppoSAX and its astrophysical implications

We report on results of a Target-of-Opportunity observation of the X-ray transient XTE J1118+480 performed on 2000 April 14-15 with the Narrow Field Instruments (0.1-200 keV) of the BeppoSAX satellite. The measured spectrum is a power law with a photon index of similar to1.7 modified by an ultrasoft X-ray excess and a high-energy cutoff above similar to 100 keV. The soft excess is consistent with a blackbody with a temperature of similar to 40 eV and a low flux, while the cutoff power law is well fitted by thermal Comptonization in a plasma with an electron temperature of similar to 10(2) keV and an optical depth of order unity. Consistent with the weakness of the blackbody, Compton reflection is weak. Although the data are consistent with various geometries of the hot and cold phases of the accreting gas, we conclude that a hot accretion disk is the most plausible model. The Eddington ratio implied by recent estimates of the mass and the distance is similar to 10(-3), which may indicate that advection is probably not the dominant cooling mechanism. We finally suggest that the reflecting medium has a low metallicity, consistent with the location of the system in the halo.