A Black Hole of > 6 Solar Masses in the X-ray Nova XTE J1118+480

Observations of the quiescent X-ray nova XTE J1118+480 with the new 6.5-m MMT have revealed that the velocity amplitude of the dwarf secondary is 698 +/- 14 km/s and the orbital period of the system is 0.17013 +/- 0.00010 d. The implied value of the mass function, f(M) = 6.00 +/- 0.36 solar masses, provides a hard lower limit on the mass of the compact primary that greatly exceeds the maximum allowed mass of a neutron star. Thus we conclude that the compact primary is a black hole. Among the eleven dynamically established black-hole X-ray novae, the large mass function of XTE J1118+480 is rivaled only by that of V404 Cyg. We estimate that the secondary supplies 34% +/- 8% of the total light at 5900A and that its spectral type is in the range K5V to M1V. A double-humped I-band light curve is probably due to ellipsoidal modulation, although this interpretation is not entirely secure because of an unusual 12-minute offset between the spectroscopic and photometric ephemerides. Assuming that the light curve is ellipsoidal, we present a provisional analysis which indicates that the inclination of the system is high and the mass of the black hole is correspondingly modest. The broad Balmer emission lines (FWHM = 2300-2900 km/s) also suggest a high inclination. For the range of spectral types given above, we estimate a distance of 1.8 +/- 0.6 kpc.