Cygnus X-1’s Stellar-Mass Black Hole is More Massive than Astronomers Thought

A black hole in the Cygnus X-1 binary system is so massive that it challenges current stellar evolution models.

An artist’s impression of the Cygnus X-1 binary system. Image credit: International Centre for Radio Astronomy Research.

Discovered in 1964, Cygnus X-1 is an X-ray binary system located in the constellation Cygnus.

The primary star, HD 226868, is a hot supergiant revolving about an unseen compact companion with a period of 5.6 days.

The companion is a so-called stellar-mass black hole, a class of black holes that comes from the collapse of a massive star.

Cygnus X-1 was the focus of a famous scientific wager between physicists Stephen Hawking and Kip Thorne, with Hawking betting in 1974 that it was not a black hole. Hawking conceded the bet in 1990.

“Stars lose mass to their surrounding environment through stellar winds that blow away from their surface,” said Professor Ilya Mandel, an astrophysicist at Monash University and the ARC Centre of Excellence in Gravitational Wave Discovery (OzGrav).

“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,” he added.

“Incredibly, it’s orbiting its companion star — a supergiant — every five and a half days at just one-fifth of the distance between the Earth and the Sun.”

Professor Mandel and colleagues observed Cygnus X-1 using the Very Long Baseline Array (VLBA).

Using the new VLBA data and archival observations, they refined the distance to the binary system and found it to be farther away than previously estimated, thus raising the inferred mass of the system’s black hole to 21.2 solar masses.

“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,” said Professor James Miller-Jones, an astronomer at Curtin University and the International Centre for Radio Astronomy Research (ICRAR).

“This method and our new measurements show the system is further away than previously thought, with a black hole that’s significantly more massive.”

“These new observations tell us the black hole is more than 20 times the mass of our Sun — a 50% increase on previous estimates,” Professor Mandel said.

“Using the updated measurements for the black hole’s mass and its distance away from Earth, we were able to confirm that Cygnus X-1 is spinning incredibly quickly — very close to the speed of light and faster than any other black hole found to date,” said Xueshan Zhao, a Ph.D. candidate studying at the National Astronomical Observatories of the Chinese Academy of Sciences.

The findings were published in the journal Science.

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