Detection of superhumps in XTE J1118+480 approaching quiescence

Zurita, C.; Casares, J.; Shahbaz, T.; Wagner, R. M.; Foltz, C. B.; Rodríguez-Gil, P.; Hynes, R. I.; Charles, P. A.; Ryan, E.; Schwarz, G.; Starrfield, S. G.
Bibliographical reference

Monthly Notices of the Royal Astronomical Society, Volume 333, Issue 4, pp. 791-799.

Advertised on:
7
2002
Number of authors
11
IAC number of authors
4
Citations
68
Refereed citations
62
Description
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 0.5mag 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~52d. 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 ~500kms-1 amplitude. We interpret this as the first spectroscopic evidence of an eccentric precessing disc.