The black hole in binary star system Cygnus X-1 has, according to new measurements, a mass of not 15, but 21 solar masses. This considerably larger mass goes against the current star evolution models. Article link https://www.news1.news/news/2021/02/first-discovered-black-hole-turns-out-to-be-heavier-than-expected.html
The result is also good news. Thanks to the measurement, the gap between the light black holes and the medium black holes – which are found with gravitational wave detectors – is narrowing.
Cygnus X-1 consists of a black hole and an accompanying star, which orbit each other. Article link https://www.news1.news/news/2021/02/first-discovered-black-hole-turns-out-to-be-heavier-than-expected.html
The black hole sucks in the stellar wind that blows out the companion star. This releases radiation that can be measured from Earth.
In 1964, Cygnus X-1 was discovered when Geiger counters caught X-rays aboard a missile. For a long time it was uncertain whether that radiation came from a black hole. In 1974, the famous physicists Stephen Hawking and Kip Thorne even made a bet on it. Hawking bet that Cygnus X-1 did not contain a black hole. In 1990 he admitted that he had lost.
Distance to Cygnus X-1
For the new mass determination of the black hole, done by an international team of astronomers, the distance to Cygnus X-1 was first measured. This has been done with the Very Long Baseline Array (VLBA), a collection of ten radio telescopes spread across the United States. They used this to observe Cygnus X-1 for six days.
The range finding method is based on an effect called parallax. “If you hold your thumb in front of you and look at it with your right eye and then only your left eye, it looks like your thumb is moving relative to the background,” explains Sera Markoff. She is a professor at the University of Amsterdam and one of the authors of the study. ‘If you measure the angle at which the thumb seems to move and know the distance between your eyes, you can calculate the distance to your thumb. In our case, our thumb is the radio emission from Cygnus X-1, the distant background object is a quasar billions of light years away, and our eyes are the radio telescopes on Earth as the Earth moves around the sun. ‘
Further and heavier
The VLBA measurements showed that Cygnus X-1 is not at 6100, but 7200 light-years away. “When our radio measurements showed that the distance was greater than we thought, we knew that the masses of the black hole and its companion star must also be larger than we thought,” says astronomer Phil Uttley of the University of Amsterdam.
The astronomers determined the mass with Newton’s laws of gravity. Cygnus X-1 appears to consist of a black hole with 21 solar masses and a star with 41 solar masses.
The fact that the black hole is so much more massive is inconsistent with current models of stellar evolution, which describe how massive stars collapse into a black hole at the end of their lives. According to the models, heavy stars lose part of their mass in the form of stellar wind.
The black hole in Cygnus X-1 likely originated from a star about 60 times the mass of the Sun, which collapsed tens of thousands of years ago. Article link https://www.news1.news/news/2021/02/first-discovered-black-hole-turns-out-to-be-heavier-than-expected.html
The fact that the black hole is heavier than expected may mean that the star lost less mass to stellar wind than the models predict. “So the stellar winds may be less efficient than we thought,” says Markoff.
Close (black) hole
The 21 solar mass black hole also helps to close the gap between light black holes – found in binary star systems like Cygnus X-1 – and medium black holes. Gravitational wave detections only show merging medium black holes of more than 30 to 40 solar masses. In binary systems such as Cygnus X-1, only black holes less than 20 times the mass of the Sun have been found so far. Uttley: ‘But how do those medium-sized black holes come about? Cygnus X-1 may show the intermediate phase, with one heavier black hole and one massive star. Ultimately they can merge into a medium-sized black hole. ‘
“Probably theorists examining massive stars will update their models in the light of these results to see what the effects are,” says Markoff. “It’s hard to say what the wider impact of this will be, but it will be interesting to see what happens.”
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