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The spectroscopic mass ratio of the black hole binary XTE J1118+480

ATel #67; Jerome A. Orosz (Utrecht University)
on 3 May 2001; 11:34 UT
Credential Certification: Jerome A. Orosz (orosz@astro.psu.edu)

Subjects: Optical, Binary, Black Hole, Transient

Referred to by ATel #: 383, 386

I obtained spectra of the black hole binary XTE J1118+480 with the 4.2 meter William Hershel Telescope on La Palma February 16/17, 2001. The instrumental configuration was the red arm of ISIS and the R600R grating, which yielded wavelength coverage from 5850 to 6650 Angstroms at a resolution of 1.55 Angstroms FWHM. A total of 22 useful spectra were obtained. From the radial velocities I find K_2 = 704 +/- 11 km/sec and T_0 = HJD 2,451,957.4278 +/- 0.0005. This value of T_0, which is the time of the closest approach of the secondary star, and the K-velocity are both consistent with previous results (McClintock et al., 2001, ApJ, 551, L147; Wagner et al., 2001, astro-ph/0104032). The mean rotational velocity of the secondary star is 114 +/- 4 km/sec, which implies a mass ratio of Q = M_bh/M_sec = 27 +/- 5 (q = M_sec/M_bh = 0.037 +/- 0.007), assuming the star is rotating synchronously with the orbit. This mass ratio, although extreme, is not nearly as extreme as the mass ratio of q = 0.008 (Q = 125) derived by Casares et al. (IAU Circular 7617) from a relation between the superhump period excess vs. the mass ratio. This large difference between the predicted and observed mass ratio may mean that the 2:1 resonance is the dominant tidal instability as suggested by Casares et al. (IAU Circular 7617) and that this resonance has a different period-excess-versus-mass ratio relation.