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 2000 April 7 to July 4. The spectrum is characterized by weak, broad, double-peaked Balmer and He lines on top of a blue continuum of slope p ≈ , as expected for an optically thick accretion disk. The weak Bowen blend seen in our spectra may indicate a low metallicity for the source. The presence of a partial S-wave pattern in the He II λ4686 line appears consistent with the reported photometric 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 and an inclination i ≳ 70° for a neutron star primary or 30° ≲ i ≲ 50° for a black hole primary with a mass between 4 and 10 M☉. Ca II absorption features observed at very high resolution constrain the interstellar hydrogen absorption column log [N(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.