We compare ultraviolet (UV) spectra of the recent soft X-ray transients XTE J1118+480 and XTE J1859+226. The emission line strengths in XTE J1118+480 strongly suggest that the accreting material has been CNO processed. We show that this system must have come into contact with a secondary star of about 1.5 M⊙, and an orbital period ∼ 15 h, very close to the bifurcation value at which the nuclear and angular momentum loss time-scales are similar. Subsequent evolution to the current period of 4.1 h was driven by angular momentum loss. In passing through a period of 7.75 h the secondary star would have shown essentially normal surface abundances. XTE J1118+480 could thus represent a slightly later evolutionary stage of A0620-00. We briefly discuss the broad Ly α absorption wings in XTE J1118+480.
Publication Source (Journal or Book title)
Monthly Notices of the Royal Astronomical Society
Haswell, C., Hynes, R., King, A., & Schenker, K. (2002). The ultraviolet line spectrum of the soft X-ray transient XTE J1118+480: A CNO-processed core exposed. Monthly Notices of the Royal Astronomical Society, 332 (4), 928-932. https://doi.org/10.1046/j.1365-8711.2002.05369.x