Chandra measurement of the geometrical distance to Cyg X-3 using its X-ray scattering halo

Using Chandra-HETGS data of Cyg X-3 we succeeded for the first time in applying a method, developed 27 years ago, to directly determine the geometric distance to X-ray sources. The method implies the existence of a halo of radiation scattered on interstellar dust. Any intensity variations of the source itself appear delayed and smeared out in the halo. By analysing and correlating the X-ray lightcurves at different radial distances from the source, we could determine the distance to Cyg X-3 to be approximately 9 kpc. Even though the statistics in this dataset are meager the usefulness and the possibilities of this method are convincingly demonstrated.