Many black holes observed so far exist in binary systems, locked in a gravitational dance with a companion star, neutron star, or another black hole. However, a new study published in Nature reveals a more complex arrangement: a black hole triple.
This groundbreaking discovery, made by physicists at MIT and Caltech, centers around V404 Cygni, a well-studied black hole located 8,000 light years from Earth. V404 Cygni was already known to have a close companion star that it is slowly devouring. However, the researchers noticed a second, much fainter blob of light in astronomical images. This second light source, they determined, was a star orbiting the black hole at a staggering distance of 3,500 astronomical units (AU) – 3,500 times the distance between the Earth and the sun.
“The fact that we can see two separate stars over this much distance actually means that the stars have to be really very far apart,” says study author Kevin Burdge, a Pappalardo Fellow in the MIT Department of Physics.
By analyzing data from the Gaia satellite, which tracks the movements of stars, the team confirmed that both stars moved in sync with each other, indicating a gravitational link to the black hole. The odds of this synchronized movement being a coincidence are astronomically low – about one in 10 million.
“It’s almost certainly not a coincidence or accident,” Burdge says. “We’re seeing two stars that are following each other because they’re attached by this weak string of gravity. So this has to be a triple system.”
The existence of this distant companion star challenges the prevailing theory that black holes form from supernovae – violent explosions of dying stars. The immense energy released during a supernova would have likely ejected a loosely bound star like the outer companion.
Instead, the researchers propose that this black hole formed through a gentler process called “direct collapse,” where a star collapses in on itself without a final explosive burst. This gentler process would leave distant objects relatively undisturbed.
To test their hypothesis, the team ran thousands of simulations, modeling different black hole formation scenarios. The results overwhelmingly favored direct collapse as the most likely explanation for the triple system’s configuration.
Furthermore, the outer star’s evolutionary stage, transitioning into a red giant, allowed the team to estimate the system’s age at around 4 billion years old. This marks the first time scientists have been able to determine the age of an ancient black hole system.
“We’ve never been able to do this before for an old black hole,” Burdge says. “Now we know V404 Cygni is part of a triple, it could have formed from direct collapse, and it formed about 4 billion years ago, thanks to this discovery.”
This discovery not only provides evidence for a less violent pathway for black hole formation but also opens up the possibility of discovering more black hole triples lurking in the cosmos.
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