High-resolution parallax measurements of Scorpius X-1

The results of eight VLBA observations at 5 GHz, spanning 3 yr, have yielded a measured trigonometric parallax for Sco X-1 of 0 ".00036 +/- 0 ".00004; 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 x 10(38) ergs s(-1) and determined distance continue to support the hypothesis that Z-source low-mass X-ray binary systems, like Sco X-l, radiate at the Eddington luminosity at a particular point in their X-ray color-color diagram.