THE TRIGONOMETRIC PARALLAX OF CYGNUS X-1

Abstract

We report a direct and accurate measurement of the distance to the X-ray\nbinary Cygnus X-1, which contains the first black hole to be discovered. The\ndistance of 1.86 (-0.11,+0.12) kpc was obtained from a trigonometric parallax\nmeasurement using the Very Long Baseline Array. The position measurements are\nalso sensitive to the 5.6 d binary orbit and we determine the orbit to be\nclockwise on the sky. We also measured the proper motion of Cygnus X-1 which,\nwhen coupled to the distance and Doppler shift, gives the three-dimensional\nspace motion of the system. When corrected for differential Galactic rotation,\nthe non-circular (peculiar) motion of the binary is only about 21 km/s,\nindicating that the binary did not experience a large "kick" at formation.\n