The well-studied V404 Cygni, an X-ray binary hosting a low-mass black hole, is part of a wide hierarchical triple with a tertiary companion at least 3,500 astronomical units (AU) away from the inner binary, according to MIT astrophysicists.
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V404 Cygni. Image credit: Burdge et al., doi: 10.1038/s41586-024-08120-6.
V404 Cygni is located about 7,800 light-years away in the constellation of Cygnus.
This system first jumped to prominence during a nova explosion more than 80 years ago, in 1938.
It also produced eruption in 1989, which was discovered by the Japanese X-ray satellite Ginga and high-energy instruments on board the Mir space station.
Known as Nova Cygni 1989, the 1989 outburst was crucial in the study of black holes.
Until then, astronomers knew only a handful of objects that they thought could be black holes, and V404 Cygni was one of the most convincing candidates.
V404 Cygni is known to host a central stellar-mass black hole in the act of consuming a small star that’s spiraling in very close to the black hole, every 6.5 days — a configuration similar to most binary systems.
But, according to the new study, a second star is circling the black hole, though at a much greater distance.
“We think most black holes form from violent explosions of stars, but this discovery helps call that into question,” said MIT researcher Kevin Burdge.
“This system is super exciting for black hole evolution, and it also raises questions of whether there are more triples out there.”
An artist’s impression of V404 Cygni: the central black hole (black dot) is in the process of consuming a nearby star (orange body at left), while a second star (upper white flash) orbits at a much farther distance. Image credit: Jorge Lugo.
Dr. Burdge and colleagues estimate the third companion is orbiting the V404 Cygni black hole every 70,000 years.
That the black hole seems to have a gravitational hold on an object so far away is raising questions about the origins of the black hole itself.
Black holes are thought to form from the violent explosion of a dying star — a process known as a supernova, by which a star releases a huge amount of energy and light in a final burst before collapsing into an invisible black hole.
The team’s discovery, however, suggests that if the newly-observed black hole resulted from a typical supernova, the energy it would have released before it collapsed would have kicked away any loosely bound objects in its outskirts.
The second, outer star, then, shouldn’t still be hanging around.
Instead, the authors suspect the V404 Cygni black hole formed through a more gentle process of direct collapse, in which a star simply caves in on itself, forming a black hole without a last dramatic flash.
Such a gentle origin would hardly disturb any loosely bound, faraway objects.
Because V404 Cygni includes a very far-off star, this suggests the system’s black hole was born through a gentler, direct collapse.
And while astronomers have observed more violent supernovae for centuries, the triple system could be the first evidence of a black hole that formed from this more gentle process.
In addition to giving clues to the black hole’s origins, the outer star has also revealed the system’s age.
The astrophysicists observed that the outer star happens to be in the process of becoming a red giant — a phase that occurs at the end of a star’s life.
Based on this stellar transition, they determined that the outer star is about 4 billion years old.
Given that neighboring stars are born around the same time, they conclude that the binary component is also 4 billion years old.
“We’ve never been able to do this before for an old black hole,” Dr. Burdge said.
“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.”
The findings were published this week in the journal Nature.
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K.B. Burdge et al. The black hole low-mass X-ray binary V404 Cygni is part of a wide triple. Nature, published online October 23, 2024; doi: 10.1038/s41586-024-08120-6
