The halo black hole X-ray transient XTE J1118 + 480

Optical spectra were obtained of the optical counterpart of the high-latitude (b ≃ 62°) soft X-ray transient XTE J1118 + 480 near its quiescent state (R ≃ 18.3) with the new 6.5 m Multiple Mirror Telescope and the 4.2 m William Herschel Telescope. The spectrum exhibits broad, double-peaked emission lines of hydrogen (FWHM ≃ 2400 km s^-1) arising from an accretion disk superposed with absorption lines of a late-type secondary star. Cross-correlation of the 27 individual spectra with late-type stellar template spectra reveals a sinusoidal variation in radial velocity with amplitude K = 701 ± 10 km s^-1 and orbital period P = 0.169930 ± 0.000004 days. The mass function, 6.1 ± 0.3 M_⊙, is a firm lower limit on the mass of the compact object and strongly implies that it is a black hole. We estimate the spectral type of the secondary to be K7 V-M0 V, and that it contributes 28% ± 2% of the light in the 5800-6400 Å region on 2000 November 20, increasing to 36% ± 2% by 2001 January 4 as the disk faded. Photometric observations (R-band) with the Instituto de Astrofísica de Canarias 0.8 m telescope reveal ellipsoidal light variations of full amplitude 0.2 mag. Modeling of the light curve gives a large mass ratio (M₁/M₂ ∼ 20) and a high orbital inclination (i = 81° ± 2°). Our combined fits yield a mass of the black hole in the range M¹ = 6.0-7.7 M_⊙ (90% confidence) for plausible secondary star masses of M₂ = 0.09-0.5 M_⊙. The photometric period measured during the outburst is 0.5% longer than our orbital period and probably reflects superhump modulations, as observed in some other soft X-ray transients. The estimated distance is d = 1.9 ± 0.4 kpc, corresponding to a height of 1.7 ± 0.4 kpc above the Galactic plane. The spectroscopic, photometric, and dynamical results indicate that XTE J1118 + 480 is the first firmly identified black hole X-ray system in the Galactic halo.