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 \pm 14 km s ^ { -1 } and the orbital period of the system is 0.17013 ~ { } \pm~ { } 0.00010 d. The implied value of the mass function , f ( M ) = 6.00 ~ { } \pm~ { } 0.36 M _ { \odot } , provides a hard lower limit on the mass of the compact primary that greatly exceeds the maximum allowed mass of a neutron star ( \sim 3 M _ { \odot } ) .
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 % \pm 8 % of the total light at 5900 Å 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 ( M _ { 1 } \lesssim 10 M _ { \odot } ) .
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 1.8 \pm 0.6 kpc .