Black hole is even more massive than we thought

New observations of the first black hole ever detected have revealed that it is in fact the most massive stellar-mass black hole ever detected without the use of gravitational waves, leading astronomers to question what they know about the universe’s most mysterious objects.

Cygnus X-1 is one of the closest black holes to Earth, and was discovered in 1964 when a pair of Geiger counters were carried on board a suborbital rocket launched from New Mexico. Now an international team of astronomers have measured the size of the black hole by using the Very Long Baseline Array — a continent-sized radio telescope made up of 10 dishes spread across the United States — together with a clever technique to measure distances in space. Their results have been published in the journal Science.

“If we can view the same object from different locations, we can calculate its distance away from us by measuring how far the object appears to move relative to the background,” said lead researcher Professor James Miller-Jones, from Curtin University and the Perth-based International Centre for Radio Astronomy Research (ICRAR).

“If you hold your finger out in front of your eyes and view it with one eye at a time, you’ll notice your finger appears to jump from one spot to another. It’s exactly the same principle.

“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 that’s significantly more massive.”

Study 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 it’s actually challenging how astronomers thought they formed.

“Stars lose mass to their surrounding environment through stellar winds that blow away from their surface,” he said. “But to make a black hole this heavy, we need to dial down the amount of mass that bright stars lose during their lifetime.

“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, 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.

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

Study co-author Xueshan Zhao, a PhD candidate studying at the National Astronomical Observatories of China (NAOC), said, “Using the updated measurements for the black hole’s mass and its distance away from Earth, I was 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.”

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

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