Far Ultraviolet Spectroscopic Explorer spectra of the black hole binary LMC X-3

Far-ultraviolet spectra of LMC X-3 were taken covering photometric phases 0.47-0.74 in the 1.7 day orbital period of the black hole binary (phase zero being superior conjunction of the X-ray source). The continuum is faint and flat but appears to vary significantly during the observations. Concurrent RXTE/ASM observations show the system was in its most luminous X-ray state during the Far Ultraviolet Spectroscopic Explorer (FUSE) observations. The far-UV spectrum contains strong terrestrial airglow emission lines, while the only stellar lines clearly present are emissions from the O VI resonance doublet. Their flux does not change signi. cantly during the FUSE observations. These lines are modeled as two asymmetric profiles, including the local ISM absorptions due to C II and possibly O VI. Velocity variations of O VI emission are consistent with the orbital velocity of the black hole and provide a new constraint on its mass.