Far Ultraviolet Spectroscopic Explorer Observations of RX J0513.9−6951

Far Ultraviolet Spectroscopic Explorer (FUSE) observations were obtained in 2003 July during 1.2 cycles of the 0.76 day binary orbit of RX J0513.9-6951. Radial velocity measurements of the broad O VI emission profile show a semiamplitude of K ~ 26 km s-1, which is much smaller than the value of 117 km s-1 measured from 2001 FUSE data. Narrow O VI emissions show no measurable velocity variation. The mean velocity of the broad O VI emission is redshifted by ~500 km s-1 with respect to both the systemic and narrow emission-line velocities. Spectral difference plots show phase-related changes in the broad emission profile. Other phase-related changes, such as line and continuum variations, are also smaller than those in the 2001 spectra. We describe a moving broad absorption feature near 1020 A as a possible O VI outflow associated with a precessing jet. We discuss the implications for the stellar masses if the 2003 broad O VI velocities outline the compact star's orbital motion.