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C. F. Bradshaw, B. J. Geldzahler (Institute for Computational Sciences and Informatics, Center for Earth Observing and Space Research, George Mason University), E. B. Fomalont (National Radio Astronomy Observatory)
We have measured the trigonometric parallax of Sco X-1 as 0''\hss.00036 ± 0''\hss.00004 which corresponds to a distance of 2.8 ± 0.3 kpc. This distance is 40% farther away than previous estimates based on X-ray luminosity. We have observed this z-source with the RXTE in the horizontal, normal, and flaring branches and at the normal-flaring branch vertex. Our measured luminosity of 2.3 \times 1038 \: ergs \: s-1 at the normal-flaring branch vertex and determined distance supports the hypothesis that Sco X-1 radiates at the Eddington luminosity at this point in its X-ray color-color diagram.
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