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Astronomers revisited the very first stellar-mass black hole ever identified and found it to be at least 50% larger than we thought.
The black hole in the Cygnus X-1 x-ray binary system has been recalculated to clock at 21 times the mass of the Sun. This makes it the most massive stellar mass black hole ever detected without the use of gravitational waves, and it is forcing astronomers to rethink how black holes are formed.
Cygnus X-1 was first discovered as an X-ray source in 1964, and its status as a black hole has continued to be the subject of a bet between astrophysicists Stephen Hawking and Kip Thorne.
The scientists then validated the black hole’s interpretation of the object’s nature, concluding that the X-ray emission was produced by the black hole munching on a binary companion.
It became one of the most studied black holes in the sky, and astronomers thought it was pretty well understood: an object about 6,070 light-years away, with a mass of 14.8 solar masses, and a blue supergiant binary companion named HDE 226868. at about 24 solar masses.
We were, according to new observations, wrong.
Astronomers made new parallax observations of the system, observing how it appears to “ sway ” across the sky as the Earth revolves around the Sun, using the Very Long Baseline Array, a collection of radio telescopes that act together like a dish collection the size of a continent.
Ultimately, their observations showed that Cygnus X-1 is quite a distance farther away than we thought. Which means that the objects themselves are significantly larger.
“We used radio telescopes to take high-precision measurements of Cygnus X-1 – the first black hole ever to be discovered,” said astronomer James Miller Jones of the International Radio Astronomical Research Center (ICRAR) in Australia.
“The black hole is in an orbit of a few days with a huge companion star. By tracking the black hole’s orbit across the sky for the first time, we have fine-tuned the distance to the system, placing it more than 7,000 light years from Earth.
“This implied that the black hole was more than 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. “
Previously, the most massive stellar mass black hole detected electromagnetically was the M33 X-7, clocked at 15.65 times the mass of the Sun. At the time of its discovery, even the M33 X-7 challenged our black hole formation models.
Scientists concluded that when the massive star that would collapse to form the black hole reached the end of its life, it lost mass more slowly than models suggested. They believe something similar for Cygnus X-1.
“Stars lose mass in their surrounding environment due to stellar winds blowing across their surface. But to make a black hole this heavy, we need to reduce the amount of mass bright stars lose in their lifetime,” said theoretical astrophysicist Ilya Mandel of the ARC Center of Excellence for the Discovery of Gravitational Waves (OzGrav) in Australia.
The black hole precursor star Cygnus X-1 is said to have started at around 60 solar masses, detonating its outer material before the core likely collapsed directly into the dense object it is today, bypassing a supernova explosion.
Now he’s locked in an incredibly close 5.6-day orbital dance with his blue supergiant companion, who now also has revised mass, bringing him to 40 solar masses.
It’s massive enough that it eventually turns into a black hole, forming a binary black hole similar to those seen in mergers that generate gravitational waves.
The binary is unlikely to merge anytime soon, however. The refined distance measurement will also allow astronomers to recalculate other features of the Cygnus X-1. In another article, astronomers discovered that it spins almost as fast as the speed of light. It’s faster than any other black hole ever measured.
This is in direct contrast to gravitational wave binaries, which have very slow or misaligned spins. This suggests that Cygnus X-1 followed a different evolutionary path than the black hole binaries we’ve seen merge.
Given the distance between Cygnus X-1 and HDE 226868, the researchers calculated that the pair is unlikely to merge within a timeframe equal to the age of the Universe – 13.8 billion years.
Studying the system now, before this second black hole collapse, presents a rare opportunity to understand black hole binaries.
“Observations like these tell us directly a lot about possible evolutionary pathways to create double black holes, which some ground-based gravitational wave detectors like LIGO and Virgo have consistently found,” said physicist Ashley Ruiter of the ‘University of New South Wales Canberra in Australia, which was not involved in the research.
“It’s great that we can still capture the binary ‘in action’ with electromagnetic light before it forms a double black hole – it helps refine our theories on the narrow evolution of binary stars.
The team’s research has been published in Science.
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