High-Resolution Parallax Measurements of Scorpius X-1

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Abstract
The results of eight VLBA observations at 5 GHz, spanning 3 yr, have yielded a measured trigonometric parallax for Sco X-1 of 000036±000004; hence, its distance is 2.8±0.3 kpc. This is the most precise parallax measured to date. Although our measured distance is 40% farther away than previous estimates based on X-ray luminosity, our Rossi X-Ray Timing Explorer observations, with a measured luminosity of 2.3×1038 ergs s-1, and determined distance continue to support the hypothesis that Z-source low-mass X-ray binary systems, like Sco X-1, radiate at the Eddington luminosity at a particular point in their X-ray color-color diagram.