Astronomers have discovered the fastest black hole spinning to date.


Pulling material from a massive blue companion star is the black hole Cygnus X-1. An accretion disc around the black hole is created by that "stuff". (Image credit: NASA/CXC)

The first black hole ever found, six decades after its discovery, is still causing astronomers to scratch their heads. The celestial behemoth at the core of the Cygnus X-1 system turns out to be 50 percent more massive than previously believed, making it the heaviest black hole stellar-mass ever directly observed.

An international team of researchers estimates that the black hole is 21 times the mass of our sun and that it spins faster than any other known black hole, based on recent observations. The recalculated weight leads scientists to reconsider the evolution of bright stars turning into black holes and how easily they lose their skin before they die.

A black hole's mass depends on its parent star's properties, such as the mass of the star and its metallicity (how much of it is made up of elements heavier than helium). It sheds its outer layers by blasts of stellar winds over a star's lifespan. Scientists think that larger stars rich in heavy elements shed their mass faster than smaller stars with less metallicity.

Stars, by stellar winds that blow away from their surface, lose mass to their surrounding atmosphere. But we need to turn down the amount of mass that bright stars lose over their lifetimes in order to make a black hole this heavy and spinning so fast, research co-author Ilya Mandel, an astrophysicist said in a statement.

Distance matters

In the new research, scientists measured the mass of Cygnus X-1 using a tried-and-tested approach called parallax to calculate the distances of stars from Earth. As Earth orbits the sun, astronomers measure the apparent motion of stars relative to the backdrop of more distant stars, and they can use that motion to determine the distance of the star from Earth with a bit of trigonometry.

Moreover, the black hole of Cygnus X-1 is slowly devouring its bright blue companion star by pulling in the outer layers of that star, creating a bright disc revolving around the black hole. It gets heated to millions of degrees and releases brilliant X-ray radiation as the matter falls into the black hole. The black hole barely avoids some of this matter and is spit out in strong jets that emit radio waves observable on Earth.

Using observations from the Very Long Baseline Array (VLBA) , a continent-size network of 10 radio telescopes distributed throughout the United States, ranging from Hawaii to the Virgin Islands, the research team monitored these signature bright jets. They observed the complete orbit of the black hole around its companion star over a period of six days and calculated how the black hole moved in space.

They discovered that Cygnus X-1 is roughly 7,200 light-years from Earth, surpassing the previous 6,000 light-year estimate. The revised distance shows that the blue supergiant companion star, 40 times more massive than our sun, is brighter and more massive than previously assumed. And they were able to give a new estimate of the mass of the black hole, a whopping 21 solar masses, given the orbital length of the black hole.

Using the revised measurements for the mass of the black hole and its distance from Earth, we were able to confirm that Cygnus X-1 spins extremely fast, very close to the speed of light and faster than any other black hole discovered to date, the study co-author Lijun Gou, a researcher at (NAOC) National Astronomical Observatories, said in the release.

The discovery is a testament to how advances in telescope sensitivity and accuracy can uncover mysteries in even some of our universe's most studied pieces.

As the next generation of telescopes comes online, their enhanced sensitivity shows the universe in increasingly more detail, said in a statement study co-author Xueshan Zhao, a NAOC researcher. Becoming an astronomer is a great time.

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