A black hole greater than 6 M⊙ in the X-ray nova XTE J1118+480

Observations of the quiescent X-ray nova XTE J1118+480 with the new 6.5 m Multiple Mirror Telescope have revealed that the velocity amplitude of the dwarf secondary is 698 +/- 14 km s(-1) and the orbital period of the system is 0.17013 +/- 0.00010 days. The implied value of the mass function, f(M) = 6.00 +/- 0.36 M., provides a hard lower limit on the mass of the compact primary that greatly exceeds the maximum allowed mass of a neutron star (similar to3 M.). Thus, we conclude that the compact primary is a black hole. Among the 11 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 5900 Angstrom and that its spectral type is in the range from K5 V to M1 V. 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 that indicates that the inclination of the system is high and the mass of the black hole is correspondingly modest (M-1 less than or similar to 10 M.). The broad Balmer emission lines (FWHM = 2300-2900 km s(-1)) also suggest a high inclination. For the range of spectral types given above, we estimate a distance of kpc. 1.8 +/- 0.6 kpc.