The Halo Black Hole X‐Ray Transient XTE J1118+480

Optical spectra were obtained of the optical counterpart of the high latitudesoft X-ray transient XTE J1118+480 near its quiescent state with the new 6.5 mMMT and the 4.2 m WHT. The spectrum exhibits broad, double-peaked, emissionlines of hydrogen from an accretion disk superposed with absorption lines of aK7V-M0V secondary star. Cross-correlation of the 27 individual spectra withlate-type stellar template spectra reveals a sinusoidal variation in radialvelocity with amplitude K = 701 +/- 10 km/s and orbital period P = 0.169930 +/-0.4 d. The mass function, 6.1 +/- 0.3 solar masses, is a firm lower limiton the mass of the compact object and strongly implies that it is a black hole.Photometric observations (R-band) with the IAC 0.8 m telescope revealellipsoidal light variations of full amplitude 0.2 mag. Modeling gives a largemass ratio (M1/M2 ~ 20) and a high orbital inclination (i = 81 +/- 2 deg). Ourcombined fits yield a mass of the black hole in the range M1 = 6.0-7.7 solarmasses (90% confidence) for plausible secondary star masses of M2 = 0.09-0.5solar masses. The photometric period measured during the outburst is 0.5%longer than our orbital period and probably reflects superhump modulations asobserved 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 Galacticplane. The spectroscopic, photometric, and dynamical results indicate that XTEJ1118+480 is the first firmly identified black hole X-ray system in theGalactic halo.Comment: 8 pages, 3 figures, AASTeX 5.0, To Appear in the Astrophysical Journal, Part 1, Volume 556, July 20, 200