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New evidence suggests the first known black hole is larger than previously thought, which may force scientists to reconsider their understanding of how giant stars give birth to black holes.
Scientists believe that stellar-mass black holes, which contain up to a few times the mass of the sun, form when giant stars die and collapse on themselves. The first black hole ever discovered was Cygnus X-1, located in the Milky Way in the constellation Cygnus, the swan. Astronomers saw the first signs of the black hole in 1964 via gas sucked from a blue super-giant star in narrow orbit. As this gas spiraled into the black hole, it became so hot that it emitted high-energy X-rays and gamma rays that satellites could detect.
A trio of studies in 2011 suggested that Cygnus X-1 was located about 6,070 light years from Earth, but new research suggests the black hole is actually around 7,240 light years away. Because other characteristics of the object are calculated based on distance, the new calculation argues that Cygnus X-1 is a little larger than scientists realized.
Related: What happens at the center of a black hole?
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To estimate the distance of the black hole, scientists use the so-called parallax technique, which examines Cygnus X-1 against its background. “If you hold one finger at arm’s length and close one eye then the other, you will see it [your finger] appear to move from location to location relative to more distant background objects, ”James Miller-Jones, astrophysicist at Curtin University node of the International Radio Astronomical Research Center in Perth, Australia, author lead of the new study and co “Using this same idea, one can calculate how far away Cygnus X-1 was by looking at it from different vantage points as the Earth moved around the sun.”
The 2011 work analyzed the light from the black hole’s companion star to help estimate the star’s diameter. With this measurement, the researchers calculated other details of the partnership, such as the mass of the black hole, suggesting it was about 14.8 times that of the sun.
However, the 2011 research did not collect data from the black hole in a full orbit around its companion star. Without this information, previous work could not fully capture how these orbital motions might affect distance and mass estimates.
In the new study, Miller-Jones and colleagues analyzed Cygnus X-1 observations from the Very Long Baseline Array (VLBA), a giant radio telescope made up of 10 dishes scattered across the United States over the course of six 12 hours of long. Observations made on consecutive days, the researchers monitored the full orbit of the black hole.
Using the parallax technique on this new data combined with the data from 2011, the scientists discovered that the black hole was possibly further away than previously thought, about 7,240 light-years from Earth.
These new findings led the researchers to revise the movement patterns of the companion star of Cygnus X-1, which led to a new estimated mass for the black hole – about 21.2 times that of the sun. This size makes Cygnus X-1 the largest stellar-mass black hole detected to date with light observations. (Gravitational wave observatories such as LIGO that detect ripples in the fabric of space and time have detected black holes of greater stellar mass, one of which is roughly 50 times the mass of the sun.)
These results suggest that stars that form stellar-mass black holes may not lose as much material to winds as previously thought. “The mass of a black hole is determined by the size of a star at which it started,” Miller-Jones said. “Stars lose mass as winds blow across their surface, and massive stars generate more powerful winds. The most massive stars can have very powerful winds and lose a lot of mass through them before forming black holes.
The new giant size of Cygnus X-1 therefore suggests that the stars that form stellar-mass black holes may be larger than previously thought. “Previous models predicted that the most massive black hole that a massive star in our Milky Way galaxy could make should only be about 15 times the mass of the sun,” Miller-Jones said. “So finding something 21 times the mass of the sun means we have to revisit our estimates of the mass lost by these massive stars.”
Updated estimates of the black hole’s mass and distance also revealed that the object was spinning very close to the speed of light, “faster than any other black hole found to date,” said study co-author Xueshan Zhao at the Chinese Academy of Sciences in Beijing, said in a statement.
And even larger stellar mass black holes may be awaiting the attention of scientists. “Cygnus X-1 is unlikely the most massive stellar mass black hole that can be produced,” Miller-Jones said. “The question is whether we can identify them and how accurately can we measure their masses?”
Scientists detailed their findings online Feb. 18 in the journal Science. Two other articles focusing on different aspects of this work also appeared on February 18 in The Astrophysical Journal.
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