Cygnus X-1 contains a 21–solar mass black hole—Implications for massive star winds

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Science  05 Mar 2021:
Vol. 371, Issue 6533, pp. 1046-1049
DOI: 10.1126/science.abb3363

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A heavy black hole in an x-ray binary

If a black hole interacts with a binary companion star, the system emits x-rays and can form a radio jet. The masses of black holes in these x-ray binaries are all lower than those detected using gravitational waves, challenging models of black hole formation from massive stars. Miller-Jones et al. used radio astrometry to refine the distance to Cygnus X-1, a well-studied x-ray binary. They found a larger distance than previous estimates, raising the mass of the black hole in the system to 21 solar masses. The results challenge the wind mass loss rates implemented in stellar evolution models.

Science, this issue p. 1046


The evolution of massive stars is influenced by the mass lost to stellar winds over their lifetimes. These winds limit the masses of the stellar remnants (such as black holes) that the stars ultimately produce. We used radio astrometry to refine the distance to the black hole x-ray binary Cygnus X-1, which we found to be 2.220.17+0.18 kiloparsecs. When combined with archival optical data, this implies a black hole mass of 21.2 ± 2.2 solar masses, which is higher than previous measurements. The formation of such a high-mass black hole in a high-metallicity system (within the Milky Way) constrains wind mass loss from massive stars.

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