A Measurement of the Broadband Spectrum of XTE J1118+480 withBeppoSAXand Its Astrophysical Implications

We report on results of a target of opportunity observation of the X-raytransient XTE J1118+480 performed on 2000 April 14-15 with the Narrow FieldInstruments (0.1-200 keV) of the SAX satellite. The measured spectrum is apower law with a photon index of ~1.7 modified by an ultrasoft X-ray excess anda high-energy cutoff above ~100 keV. The soft excess is consistent with ablackbody with temperature of ~40 eV and a low flux, while the cut-off powerlaw is well fitted by thermal Comptonization in a plasma with an electrontemperature of 100 keV and an optical depth of order of unity. Consistent withthe weakness of the blackbody, Compton reflection is weak. Though the data areconsistent with various geometries of the hot and cold phases of the accretinggas, we conclude that a hot accretion disk is the most plausible model. TheEddington ratio implied by recent estimates of the mass and the distance isabout 10^{-3}, which may indicate that advection is probably not the dominantcooling mechanism. We finally suggest that the reflecting medium has a lowmetallicity, consistent with location of the system in the halo.Comment: 14 pages, 8 figures, accepted for publication in The Astrophysical Journa