Astronomers revisited the very first stellar-mass black hole ever identified and found it to be at least 50 percent more massive than we thought.
The black hole in the X-ray binary system Cygnus X-1 has been recalculated to clock in 21 times the mass of the sun. That makes it the most massive stellar-mass black hole ever detected without the use of gravitational waves, and it forces astronomers to rethink how black holes form.
Cygnus X-1 was first discovered as an X-ray source in 1964, and its status as a black hole became the subject of a bet between astrophysicists Stephen Hawking and Kip Thorne.
Scientists later validated the black hole’s interpretation of the object’s nature and concluded that the X-rays were produced by the black hole snacking on a binary companion.
It has become one of the most studied black holes in the sky, and astronomers thought it was fairly well understood: an object about 6,070 light-years away, with a mass of 14.8 solar masses, and a blue supergiant binary companion called HDE 226868 clocks in approximately 24 solar masses.
According to new observations, we were wrong.
Astronomers have conducted new parallax observations of the system, looking at how it appears to ‘wobble’ in the sky as the Earth revolves around the sun, using the Very Long Baseline Array, a collection of radio telescopes that together act as a collecting tray the size of a continent.
Ultimately, their observations showed that Cygnus X-1 is quite a distance away than we thought. Which means that the objects themselves are considerably larger.
“We used radio telescopes to make highly accurate measurements of Cygnus X-1 – the first black hole ever discovered,” explains astronomer James Miller Jones of the International Center for Radio Astronomy Research (ICRAR) in Australia.
“The black hole orbits a few days with a massive companion star. By tracing the black hole’s orbit in the sky for the first time, we refined its distance from the system, making it more than 7,000 light years. from the 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 about how massive stars evolve into black holes. “
Previously, the most massive stellar-mass black hole detected electromagnetically was M33 X-7, clocked at 15.65 times the mass of the sun. At the time of its discovery, even 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 to their surrounding environment due to stellar winds blowing away from their surfaces. But to make a black hole that massive, we need to reverse the amount of mass that bright stars lose during their lifetime,” theoretical astrophysicist Ilya Mandel said. from the ARC Center of Excellence in Gravitational Wave Discovery (OzGrav) in Australia.
The precursor to the Cygnus X-1 black hole is said to have started at about 60 times the mass of the Sun and ejected its outer material before likely collapsing directly into the dense object it is today, bypassing a supernova explosion.
Now it is locked in an incredibly narrow 5.6-day orbital dance with its blue supergiant companion, which now also has a revised mass, bringing it to a thick 40 solar masses.
That is so big that it would one day also end up as a black hole and form a double black hole similar to that of the mergers that generate gravitational waves.
However, it is unlikely that the binary will be merged any time soon. The refined distance measurement also allows astronomers to recalculate other features of Cygnus X-1. In a separate article, astronomers found that it spins almost as fast as the speed of light. That is faster than any other black hole ever measured.
This is in stark 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 have seen merge.
Given the distance between Cygnus X-1 and HDE 226868, the researchers calculated that the pair is unlikely to merge within a timescale equal to the age of the Universe – 13.8 billion years.
If we study the system now, before that second black hole collapse, it offers a rare opportunity to understand the black hole binaries.
“Observations like these directly tell us a lot about the evolutionary paths that are possible in creating double black holes, some of which ground-based gravitational wave detectors such as LIGO and Virgo have found regularly,” said University of New South Wales physicist Ashley Ruiter. Canberra in Australia, which was not involved in the investigation.
“It’s great that we can still see the binary star ‘in action’ with electromagnetic light before it forms a double black hole – it helps refine our theories about the evolution of nearby binary stars.”
The team’s research is published in Science