OBSERVATIONS OF SCORPIUS-X-1 WITH IUE - ULTRAVIOLET RESULTS FROM A MULTIWAVELENGTH CAMPAIGN

We present ultraviolet results from a multiwavelength campaign on the low-mass X-ray binary Sco X-1. The ultraviolet observations were obtained with the International Ultraviolet Explorer (IUE) and coincided with X-ray observations taken with the Japanese satellite Ginga and other observations from numerous between the strengths of the UV continuum and line fluxes and the X-ray spectral states. Models predicting UV continuum emission from the X-ray - heated surface of the companion star and from an X-ray - illuminated accretion disk were adjusted for parameters intrinsic to Sco X-1 and fitted to the data. The data/model comparison shows the following for Sco X-1, as for Cyg X-2: (1) X-ray heating of the accretion disk is the dominant source of UV emission, (2) the mass accretion rate increases monotonically along the so-called Z-shaped curve in an X-ray color-color diagram, and (3) the disk structure does not change as a function of X-ray spectral state. The accretion disk model gives a best-fit outer disk radius of (6.3 +/- 0.3) x 10(10) cm. Mass accretion rates range from 0.6 x 10(-8) M. yr-1 in the normal branch (NB) to 1.1 x 10(-8) M. yr-1 in the flaring branch (FB). The horizontal branch (HB) was not observed simultaneously in the UV and X-ray, but if the weakest state seen from earlier UV observations corresponds to the HB state, the mass accretion rate there is found to be 0.4 x 10(-8) M. yr-1. Ultraviolet emission lines from He, C, N, O, and Si were detected; all increase in intensity from the HB to the FB state. A model in which emission lines are due to photoionization of the outer disk by the X-ray source was developed and gives good agreement with the line fluxes observed in each state. The X-ray:UV:radio flux ratios of Sco X-1 measured at the lower vertex of the Z in the X-ray color-color diagram agree with those previously observed in five other X-ray binaries. We suggest that the flux at this vertex, which several theories identify with an accretion rate at the Eddington limit, may be used as a standard candle; the distance to Sco X-1 would then be 2.0 +/- 0.5 kpc.