First Black Hole in a Triple Star System Found

Astrophysicists from Caltech and MIT report that they have observed for the first time a "black hole triple"—a system of three stars, one of which is a black hole. Prior to the discovery, this black hole was thought to have just one partner star not two.

In this system, called V404 Cygni, the black hole is consuming a small star that is spiraling in very close and fast while a newfound third star circles the black hole from much farther away. The far-off companion is orbiting the black hole roughly every 70,000 years.

"The tertiary star has actually been hiding in plain sight for more than 30 years. Dozens of other papers have noticed that there is another star 'next to' V404 Cygni on the sky, but they just all assumed the star was a chance alignment," explains Kareem El-Badry, an assistant professor of astronomy at Caltech. "Only with precise astrometry from [the European Space Agency's] Gaia mission did it become clear that the star is actually at the same distance and is moving in the same direction as V404 Cygni, meaning that it's gravitationally bound to it."

El-Badry is second author of a new Nature study reporting the results; the first author is Kevin Burdge, a Pappalardo Fellow in the MIT Department of Physics and a Caltech alum (PhD '21).

The discovery brings up a pressing question: How does the black hole have a gravitational hold on an object so far away? A black hole is thought to form from the violent explosion of a dying star, a phenomenon known as a supernova. But when this occurs, the explosion is expected to kick away any loosely bound objects. Thus, the third star orbiting in the V404 Cygni system would not be expected to still be hanging around.

Instead, the team suspects the black hole formed through a process of implosion, in which a star would simply cave in on itself, forming a black hole without a last dramatic flash. Such a gentle origin might explain why the black hole has a distant companion.

Whether other black hole triples remain to be discovered is not clear. "Either we got very lucky, or tertiaries are common," says El-Badry. "If they are common, that might solve some of the long-standing questions about how black hole binaries form. Triples open up evolutionary pathways that are not possible for pure binaries. People have actually predicted before that black hole binaries might form mostly through triple evolution, but there was never any direct evidence until now."

Read the full story from MIT.

The study, titled "The black hole low-mass X-ray binary V404 Cygni is part of a wide triple," was funded by the National Science Foundation.