The Trigonometric Parallax of Cygnus X-1

TL;DR

This paper presents a precise measurement of the distance to Cygnus X-1 using VLBA parallax, revealing its orbit, proper motion, and minimal peculiar velocity, which informs black hole formation theories.

Contribution

It provides the first direct parallax-based distance to Cygnus X-1 and details its orbital and motion characteristics, improving understanding of black hole systems.

Findings

Distance to Cygnus X-1 is 1.86 kpc.

The binary orbit is clockwise on the sky.

Peculiar velocity is about 21 km/s, indicating minimal natal kick.

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.