Optical Spectroscopy Of The X-Ray Transient XTE J1118+480 In Outburst

Abstract

We report on optical spectroscopic observations of the X-ray Transient XTE J1118+480 covering the period from April 7, 2000 to July 4, 2000. The spectrum is characterized by weak, broad, double-peaked Balmer and He lines on top of a blue continuum of slope p ~ 1/3, as expected for an optically-thick accretion disk. The weak Bowen blend seen in our spectra may indicate a low intrinsic metallicity for the source. The presence of a partial S-wave pattern in the HeII lambda-4686 line appears consistent with the reported photometric orbital period Porb = 4.1 hr for XTE J1118+480. By using a combination of Doppler mapping and various theoretical arguments, we constrain plausible orbital parameters for the system: a mass ratio 0.02 <~ q <~ 0.1, an inclination i >~ 70 deg for a neutron star primary, or 30 deg <~ i <~ 50 deg for a black hole primary with a mass between 4 and 10 Msun. CaII absorption features observed at very high resolution constrain the interstellar hydrogen absorption column log[N_HI (cm^{-2})] ~ 20.45 +/- 0.2 and the identification of three absorbing clouds indicate a distance to the source <~ 1 kpc, assuming the line-of-sight to XTE J1118+480 has average high-latitude properties. These results are discussed in the context of previous multiwavelength observations of this unusual system.