The research shows the system known as Cygnus X-1 contains the most massive stellar-mass black hole ever detected without the use of gravitational waves.

Cygnus X-1 is a stellar-mass black hole- one of the closest black holes to Earth. Discovered in 1964, the object gathered the attention of a famous scientific wager between physicists Stephen Hawking and Kip Thorne.
In 1974, Hawking argued that Cygnus X-1 was not a black hole.
In a new study, an international team of astronomers used the Very Long Baseline Array and radio astrometry to measure distances to the black hole X-ray binary Cygnus X-1.
Their study has shown that Cygnus X-1 contains the most massive stellar-mass black hole ever detected without gravitational waves. It is more massive than previously thought.
Lead researcher, Professor James Miller-Jones from Curtin University and the International Centre for Radio Astronomy Research (ICRAR), said, “Over six days we observed a full orbit of the black hole and used observations taken of the same system with the same telescope array in 2011. This method and our new measurements show the system is further away than previously thought, with a black hole thats significantly more massive.”
Co-author Professor Ilya Mandel from Monash University and the ARC Centre of Excellence in Gravitational Wave Discovery (OzGrav) said the black hole is so massive its challenging how astronomers thought they formed.
“Stars lose mass to their surrounding environment through stellar winds that blow away from their surface. But to make a black hole this heavy, we need to dial down the amount of mass that bright stars lose during their lifetimes.”
“The black hole in the Cygnus X-1 system began life as a star approximately 60 times the mass of the Sun and collapsed tens of thousands of years ago. Incredibly, its orbiting its companion stara supergiantevery five and a half days at just one-fifth of the distance between the Earth and the Sun.”
“These new observations tell us the black hole is more than 20 times the mass of our Suna 50 percent increase on previous estimates.”
Xueshan Zhao, a co-author on the paper and a Ph.D. candidate studying at the National Astronomical Observatories, said, “Using the updated measurements for the black holes mass and its distance away from Earth, I was able to confirm that Cygnus X-1 is spinning incredibly quicklyvery close to the speed of light and faster than any other black hole found to date.”
“Im at the beginning of my research career, so being a part of an international team and helping to refine the properties of the first black hole ever discovered has been a great opportunity.”
Journal Reference:

1. James C. A. Miller-Jones et al. Cygnus X-1 contains a 21solar mass black holeImplications for massive star winds. DOI: 10.1126/science.abb3363
2. Xueshan Zhao et al. Re-estimating the Spin Parameter of the Black Hole in Cygnus X-1. DOI: 10.3847/1538-4357/abbcd6
3. Coenraad J. Neijssel et al. Wind Mass-loss Rates of Stripped Stars Inferred from Cygnus X-1. DOI: 10.3847/1538-4357/abde4a