Black hole circling in the wind

“The most exciting takeaway message is that Cygnus X-1 hosts the most massive black hole formed from the death of a star that we have ever detected without the use of gravitational waves,” says astrophysicist James Miller-Jones of the International Centre for Radio Astronomy Research at Curtin University in Perth, Australia.

Co-author David Russell at New York University Abu Dhabi contributed expertise on black holes and familiarity with Cygnus X-1.

Cygnus X-1 consists of a black hole orbiting a blue supergiant star. It releases a large amount of X-ray radiation, which first allowed it to be detected. The black hole is steadily accumulating matter and mass by drawing in particles that stream from its companion star in the process known as stellar wind.

The team used radio astrometry to refine calculation of the distance from Earth to Cygnus X-1, determining a value substantially farther away than previous estimates. This implied that the black hole must be around 21 times the mass of our sun, rather than the previous estimate of 15 solar masses.

Miller-Jones explains that a “breakthrough moment” came in a conversation with fellow senior author Ilya Mandel of Monash University in Australia. Mandel realised that prevailing estimates of the amount of mass lost by massive stars in stellar winds didn’t fit with the black hole’s revised mass. The team realised they would have to revise downwards the mass loss rate of massive stars to explain their new estimate.

“This was the key insight that led us to write this paper,” says Miller-Jones. It offers a new understanding of stellar winds that could be relevant to many other systems in the cosmos.

“This is an impressive result,” says astrophysicist Michael Nowak of Washington University in St. Louis, USA, who was not involved in the study. “It is going to make us rethink models of how massive stars lose mass to winds before they die, and how these stars eventually collapse into black holes.”

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