Black hole in rare triple system sheds light on natal kicks

For the first time, astronomers have observed a black hole in a triple system with two other stars. The system is called V404 Cygni and was previously thought to be a closely-knit binary comprising a black hole and a star. Now, Kevin Burdge and colleagues at the Massachusetts Institute of Technology (MIT) have shown that the pair is orbited by a more distant tertiary star.

The observation supports the idea that some black holes do not experience a “natal kick” in momentum when they form. This is expected if a black hole is created from the sudden implosion of a star, rather than in a supernova explosion.

When black holes and neutron stars are born, they can gain momentum through mechanisms that are not well understood. These natal kicks can accelerate some neutron stars to speeds of hundreds of kilometres per second. For black holes, the kick is expected to be less pronounced — and in some scenarios, astronomers believe that these kicks must be very small.

Information about natal kicks can be gleaned by studying the behaviour of X-ray binaries, which usually pair a main sequence star with a black hole or neutron star companion. As these two objects orbit each other, material from the star is transferred to its companion, releasing vast amounts of gravitational potential energy as X-rays and other electromagnetic radiation.

Wobbling objects

In such binaries, any natal kick the black hole may have received during its formation can be deduced by studying how the black hole and its companion star orbit each other. This can be done using the radial velocity (or wobble) technique, which measures the Doppler shift of light from the orbiting objects as they accelerate towards and then away from an observer on Earth.

In their study, Burdge’s team scrutinized archival observations of V404 Cygni that were made using a number of different optical telescopes. A bright blob of light thought to be the black hole and its close-knit companion star is prominent in these images. But the team noticed something else, a second blob of light that could be a star orbiting the close-knit binary.

“We immediately noticed that there was another star next to the binary system, moving together with it,” Burdge explains. “It was almost like a happy accident, but was a direct product of an optical and an X-ray astronomer working together.”

As Burdge describes, the study came as a result of integrating his own work in optical astronomy with the expertise of MIT’s Erin Kara, who does X-ray astronomy on black holes. Burge adds, “We were thinking about whether it might be interesting to take high speed movies of black holes. While thinking about this, we went and looked at a picture of V404 Cygni, taken in visible light.”

Hierarchical triple

The observation provided the team with clear evidence that V404 Cygni is part of a “hierarchical triple” – an observational first. “In the system, a black hole is eating a star which orbits it every 6.5 days. But there is another star way out there that takes 70,000 years to complete its orbit around the inner system,” Burdge explains. Indeed, the third star is about 3500 au (3500 times the distance from the Earth to the Sun) from the black hole.

By studying these orbits, the team gleaned important information about the black hole’s formation. If it had undergone a natal kick when its progenitor star collapsed, the tertiary system would have become more chaotic – causing the more distant star to unbind from the inner binary pair.

The team also determined that the outer star is in the later stages of its main-sequence evolution. This suggests that V404 Cygni’s black hole must have formed between 3–5::billion years ago. When the black hole formed, the researchers believe it would have removed at least half of the mass from its binary companion. But since the black hole still has a relatively low mass, this means that its progenitor star must have lost very little mass as it collapsed.

“The black hole must have formed through a gentle process, without getting a big kick like one might expect from a supernova,” Burdge explains. “One possibility is that the black hole formed from the implosion of a star.”

If this were the case, the star would have collapsed into a black hole directly, without large amounts of matter being ejected in a supernova explosion. Whether or not this is correct, the team’s observations suggest that at least some black holes can form with no natal kick – providing deeper insights into the later stages of stellar evolution.

The research is described in Nature.