Detection of superhumps in XTE J1118+480 approaching quiescence

Abstract

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 mags from December 2000 to its present near quiescent level at R=18.65 (June 2001). The ellipsoidal lightcurve is distorted by an additional modulation that we interpret as a superhump of P_sh=0.17049(1) d i.e. 0.3% longer than the orbital period. This implies a disc precession period P_prec= 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 deg. However, we urge caution in the interpretation of these values because of residual systematic contamination of the ellipsoidal lightcurve by the complex form of the superhump modulation. The orbital--mean H-alpha profiles exhibit clear velocity variations with ~500 km/s amplitude. We interpret this as the first spectroscopic evidence of an eccentric precessing disc.