First Black Hole Ever Detected is More Massive Than Thought

By: Glenys Young 

Texas Tech's Tom Maccarone was part of the international collaboration behind the discovery.

New observations of the first black hole ever detected have led astronomers to question what they know about the universe's most mysterious objects.

Published today (Feb. 18) in the journal Science, 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's black hole is one of the closest to Earth. Discovered in 1964 when a pair of Geiger counters were carried on board a sub-orbital rocket launched from New Mexico, it has enjoyed substantial fame, for a black hole. The object 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 and conceding that bet in 1990. It also was popularized in two songs by the classic rock band Rush in the late 1970s.

As much as we thought we knew about Cygnus X-1's black hole, this research highlights how much we still can learn, says study co-author Tom Maccarone, the Presidential Research Excellence Professor in Texas Tech University's Department of Physics & Astronomy.

"Cygnus X-1's was already the most massive stellar-mass black hole that had a reasonably secure mass estimate," Maccarone said. "This pushes it up even further, into a range close to where most of the merging black holes seen by gravitational waves have been found. It also has a massive companion star that may also turn into a black hole, although there is a high probability that the star will merge with the black hole before it becomes a black hole itself."

In this latest work, an international team of astronomers used 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.

"If we can view the same object from different locations, we can calculate its distance from us by measuring how far the object appears to move relative to the background," said lead researcher James Miller-Jones, a professor at Curtin University in Australia and the International Centre for Radio Astronomy Research. "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 farther away than previously thought, with a black hole that's significantly more massive."

Co-author Ilya Mandel, a professor at Australia's Monash University and the ARC Centre of Excellence in Gravitational Wave Discovery, 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 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, 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."

Next year, the world's biggest radio telescope – the Square Kilometre Array – will begin construction in Australia and South Africa, allowing for even greater study than is now possible.

"Studying black holes is like shining a light on the universe's best kept secret – it's a challenging but exciting area of research," Miller-Jones said. "As the next generation of telescopes comes online, their improved sensitivity reveals the universe in increasingly more detail, leveraging decades of effort invested by scientists and research teams around the world to better understand the cosmos and the exotic and extreme objects that exist.

"It's a great time to be an astronomer."