We present the results of our monitoring of the halo black hole soft X-ray transient (SXT) XTE J1118+480 during its decline to quiescence. The system has decayed 0.5 mag from 2000 December to its present near-quiescent level at R ≃ 18.65 (2001 June). The ellipsoidal light curve is distorted by an additional modulation that we interpret as a superhump of Psh = 0.17049(1) d i.e. 0.3 per cent longer than the orbital period. This implies a disc precession period Pprec ∼ 52 d. After correcting the average phase-folded light curve for veiling, the amplitude difference between the minima suggests that the binary inclination angle lies in the range i = 71-82°. However, we urge caution in the interpretation of these values because of residual systematic contamination of the ellipsoidal light curve by the complex form of the superhump modulation. The orbital-mean Hα profiles exhibit clear velocity variations with ∼ 500 km s-1 amplitude. We interpret this as the first spectroscopic evidence of an eccentric precessing disc.
Publication Source (Journal or Book title)
Monthly Notices of the Royal Astronomical Society
Zurita, C., Casares, J., Shahbaz, T., Wagner, R., Foltz, C., Rodríguez-Gil, P., Hynes, R., Charles, P., Ryan, E., Schwarz, G., & Starrfield, S. (2002). Detection of superhumps in XTE J1118+480 approaching quiescence. Monthly Notices of the Royal Astronomical Society, 333 (4), 791-799. https://doi.org/10.1046/j.1365-8711.2002.05450.x