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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 in the journal Science, 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 one of the closest black holes to Earth. It was discovered in 1964 when a pair of Geiger counters were carried on board a sub-orbital rocket launched from New Mexico.
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. Hawking conceded the bet in 1990.
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 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 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,” Professor Miller-Jones said. “This method and our new measurements show the system is further away than previously thought, with a black hole that’s 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 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. But to make a black hole this heavy, we need to dial down the amount of mass that bright stars lose during their lifetimes” he said.
“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 said. “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 per cent increase on previous estimates.”
Xueshan Zhao is a co-author on the paper and a PhD candidate studying at the National Astronomical Observatories—part of the Chinese Academy of Sciences (NAOC) in Beijing.
“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,” she said.
“I’m 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.”
Next year, the world’s biggest radio telescope—the Square Kilometre Array (SKA)—will begin construction in Australia and South Africa.
“Studying black holes is like shining a light on the Universe’s best kept secret—it’s a challenging but exciting area of research,” Professor 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.”
Accompanying the publication in Science, two further papers focusing on different aspects of this work have also been published today in The Astrophysical Journal.
Original Publication:
‘Cygnus X-1 contains a 21-solar mass black hole – implications for massive star winds’, published in Science on February 18th, 2021.
Companion Papers:
‘Reestimating the Spin Parameter of the Black Hole in Cygnus X-1’, published in The Astrophysical Journal on February 18th, 2021.
‘Wind mass-loss rates of stripped stars inferred from Cygnus X-1’, published in The Astrophysical Journal on February 18th, 2021.
Multimedia:
Images and animation available from www.icrar.org/biggest-black-hole
Embargo period password for access is: “Hawking”
Expert Reaction
These comments have been collated by the Science Media Centre to provide a variety of expert perspectives on this issue. Feel free to use these quotes in your stories. Views expressed are the personal opinions of the experts named. They do not represent the views of the SMC or any other organisation unless specifically stated.
Professor James Miller-Jones is the Science Director at the Curtin University node of the International Centre for Radio Astronomy Research
We used radio telescopes to make high-precision measurements of Cygnus X-1 — the first black hole ever discovered. The black hole is in a few-day orbit with a massive companion star. By tracking for the first time the black hole's orbit on the sky, we refined the distance to the system, placing it over 7000 light years from Earth. This implied that the black hole was over 20 times the mass of our Sun, making it the most massive stellar-mass black hole ever discovered without the use of gravitational waves. This challenges our ideas of how massive stars evolve to form black holes.
Through the work of astronomers and astronomical facilities around the world (and in orbit), we’re constantly furthering our knowledge of the Universe, studying exotic objects like black holes and pushing the boundaries of what’s possible. Later this year, construction of the world’s biggest radio telescope—the Square Kilometre Array (SKA)—will begin in Australia and South Africa. This 50-year project represents an enormous opportunity for engineers, scientists and big data specialists to come together to deliver a facility that will provide us with an unprecedented view of the cosmos while generating down-to-earth innovations and technologies that will benefit people’s everyday lives.
The companion is a massive star that will likely evolve into a black hole itself based on its current mass estimate. Observations like these directly tell us a lot about the evolutionary pathways that are possible in making double black holes, some of which ground-based gravitational wave detectors like LIGO and Virgo have been regularly finding.
However, it could be that when the companion forms a black hole, the black holes fly off in different directions and they would never coalesce. A system that does this will likely go undetected in gravitational waves. But it's great we can still catch the binary "in action" with electromagnetic light before it forms a double black hole - it helps to refine our theories about close binary star evolution.
Richard de Grijs is Professor of Astrophysics at Macquarie University and the Executive Director of the International Space Science Institute-Beijing.
This exciting new study has started to close the gap between run-of-the-mill stellar black holes and their counterparts producing the elusive "gravitational waves" emitted by the most violent collisions in the Universe. The conclusive detection of such violent mergers between extremely massive stars resulted in the 2017 Nobel Prize in Physics.
While the headline-grabbing black-hole mass implied for Cygnus X-1 is mind-boggling for a "normal" star, the implications are even more profound. Massive stars are notoriously difficult to understand based on our existing toolkit. This is because of the numerous uncertainties hampering the theoretical assumptions that have gone into even our best theoretical models.
This study had provided important new insights into the degree to which very massive stars lose their outer atmospheres in the form of huge outflows---known as "stellar winds". This will certainly prompt theorists and computational astrophysicists to go into overdrive!
However, don't expect fireworks any time soon... Cygnus X-1 and its binary companion star are set to grow old and faint together; a merger of cosmic proportions is as yet off the cards.
Professor Alister Graham (former ARC Future Fellow and past Director of Swinburne Astronomy Online) is a research-active astronomer at Swinburne University.
I've been wondering where our Galaxy's higher mass black holes are. With a revised 40 per cent increase in mass, the black hole in Cygnus X-1 has solidified its status as the largest known black hole in our Galaxy created from a star.
Such black holes are the compacted cores of massive stars that dramatically collapsed before their envelopes subsequently rebounded in a supernova flash. Depending on the birth mass and chemical composition of massive stars, they are thought to be capable of creating black holes up to 100 times our Sun's mass. Cygnus X-1 has a revised mass 21 times that of our Sun, more than double that of the ten other such `stellar-mass' black holes known in our Galaxy.
Moreover, it continues to feed off a companion star whose mass has also now been doubled to 40 times our Sun's mass. However, this donor star is likely to form a black hole before being fully devoured. While millions of black holes lurk in our Galaxy, the current feeding rate of Cygnus X-1 is a bit of a hot mess, generating lots of X-rays and optical light.
But don't worry, our atmosphere blocks the harmful rays. In a couple of months, and with a good pair of binoculars, I might get up before dawn and look to the east to connect with this leviathan of our Galaxy.
Dr Themiya Nanayakkara is an astronomer at Swinburne University of Technology studying the early Universe and he is the Australian point of contact for the James Webb Space Telescope user support.
With recent advancements in gravitational wave astronomy, astronomers have started to discover evidence for massive black holes exceeding 50 times the mass of our sun. However, such large black holes have not been detected so far from observations carried out by x-ray/visible/infra-red/radio telescopes and even current theoretical models find it challenging to produce such large black holes.
In this study, researchers have obtained new observations of a well-known x-ray binary system (a black hole with a companion star) and recomputed the distance to find that the mass of the black hole is 21 times the mass of our sun. This is the largest known black hole to be observed with electromagnetic radiation so far. This research provides vital clues necessary for stellar evolutionary models to develop formation mechanisms to large mass black holes.
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