The Trigonometric Parallax of Cygnus X-1

Authors:
  • MRMark J. Reid
    Mark J. Reid
  • JMJeffrey E. McClintock
    Jeffrey E. McClintock
  • RNRamesh Narayan
    Ramesh Narayan
  • LGLijun Gou
    Lijun Gou
  • RRRonald A. Remillard
    Ronald A. Remillard
  • JOJerome A. Orosz
    Jerome A. Orosz
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Abstract
We report a direct and accurate measurement of the distance to the X-ray binary Cygnus X-1, which contains the first black hole to be discovered. The distance of 1.86 (-0.11,+0.12) kpc was obtained from a trigonometric parallax measurement using the Very Long Baseline Array. The position measurements are also sensitive to the 5.6 d binary orbit and we determine the orbit to be clockwise on the sky. We also measured the proper motion of Cygnus X-1 which, when coupled to the distance and Doppler shift, gives the three-dimensional space motion of the system. When corrected for differential Galactic rotation, the non-circular (peculiar) motion of the binary is only about 21 km/s, indicating that the binary did not experience a large "kick" at formation.