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

Optical spectra were obtained of the optical counterpart of the high-latitu de (b similar or equal to 62 degrees) soft X-ray transient XTE J1118+480 ne ar its quiescent state (R similar or equal to 18.3) with the new 6.5 m Mult iple Mirror Telescope and the 4.2 m William Herschel Telescope. The spectru m exhibits broad, double-peaked emission lines of hydrogen (FWHM similar or equal to 2400 km s(-1)) arising from an accretion disk superposed with abs orption lines of a late-type secondary star. Cross-correlation of the 27 in dividual spectra with late-type stellar template spectra reveals a sinusoid al variation in radial velocity with amplitude K=701 +/- 10 km s(-1) and or bital 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 im plies that it is a black hole. We estimate the spectral type of the seconda ry to be K7 V-M0 V, and that it contributes 28% +/- 2% of the light in the 5800-6400 Angstrom region on 2000 November 20, increasing to 36% +/- 2% by 2001 January 4 as the disk faded. Photometric observations (R-band) with th e Instituto de de Canarias 0.8 m telescope reveal ellipsoidal Astrofisica l ight variations of full amplitude 0.2 mag. Modeling of the light curve give s a large mass ratio (M-1/M-2 similar to 20) and a high orbital inclination (i = 81 degrees +/- 2 degrees). Our combined fits yield a mass of the blac k hole in the range M-1 = 6.0-7.7 M-. (90% confidence) for plausible second ary star masses of M-2 = 0.09-0.5 M-.. The photometric period measured duri ng the outburst is 0.5% longer than our orbital period and probably reflect s superhump modulations, as observed in some other soft X-ray transients. T he 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, a nd dynamical results indicate that XTE J1118+480 is the first firmly identi fied black hole X-ray system in the Galactic halo.