This artist’s conception shows the V404 Cygni triple system. In the foreground are the black hole (right) and the nearby companion from which it draws mass. Near the top center, in the background, is the second newly found companion. Credit: Jorge Lugo
Despite their destructive force, black holes are often seen together with a companion, such as a star, neutron star, white dwarf, or even another black hole. However, a study published on October 23 in Nature found that the V404 Cygni black hole binary system, which contains a black hole and a small star, has another friend: a second star that is about 3,500 times farther from the “original” binary system than Earth’s distance from Sun.
This is a rare find. The team of researchers from MIT and Caltech were initially looking for signs of new black holes within the Milky Way. But then the study’s lead author Kevin Burdge, a Pappalardo Fellow at MIT, came across the third member of V404 Cyg while using Aladin Lite, an online visualization tool that consolidates several surveys. He noticed that there was an extra mass suspiciously close to the V404 Cyg system in the visible wavelengths.
The discovery now prompts astronomers to wonder about the mechanisms behind the formation of black holes, how they interact with their companions, and whether multiple triple systems exist. V404 Cyg may be the first evidence for the idea that black holes can form in a gentler and less dramatic way than traditionally thought, allowing them to keep a distant star gravitationally bound and not eject it into space.
A second spot of light
V404 Cyg has been observed since 1992 and was one of the first confirmed black holes. The previously known mass of light that Burdge saw in Aladin Lite is caused by the system’s inner star powering the black hole and creating a bright accretion disk. However, the second spot – the light from a single star – must have long been overlooked.
To confirm whether the star was related to V404 Cyg, the team used data from ESA’s Gaia search to observe the movements of known and potential companion stars over the past 10 years. The two stars appeared to be moving together relative to other nearby stars, making it clear that both are orbiting the black hole and proving that V404 Cyg is the first “triple black hole.”
The outer star is weakly bound to the black hole and its inner star. In a press release, Burdge compared it to a situation where “you’re pulling on a kite and instead of a strong string, you’re pulling with a spider web. If you pull too hard, the net will break and you will lose the kite. Gravity is like a newly tied string that’s really weak, and if you do something dramatic to the inner binary system, you’ll lose the outer star.
But astronomers have long believed that black hole formation is a violent process. So how did this outer star remain stationary, connected only by a cosmic web? This is what pushes astrophysicists to consider alternative processes of black hole formation that have never previously had support from observations.
Explosion or direct collapse?
In the early 1970s, astronomers realized that supernovae could form black holes, and this has been the accepted mechanism ever since. But when a supernova occurs, material and energy are expelled outward. This should wipe out all distant companions with only weak connections, such as V404 Cyg’s third star.
Instead, perhaps the black hole in this system suffered an unusually silent death in which the dying star imploded in what is known as a direct collapse. This does not expel energy and therefore would not eject the outer star.
To determine whether this was likely, the team ran tens of thousands of simulations to see if they could arrive at the current trinary system. Each model started with three stars, including one that dies as a supernova or undergoes a direct collapse. When the supernova exploded outward, the third star had no chance of remaining in the system. But if the star had imploded, the probability that the system would remain triple would have increased significantly.
“The vast majority of simulations show that the simplest way to accomplish this triple work is through direct collapse,” Burdge said.
Further finds
V404 Cyg’s inner companion has an orbital period of just 6.5 days. But the outer star takes as many as 70,000 years to complete a single orbit around the black hole, highlighting how loosely bound it must be. That outer star is also aging and starting to become a red giant. This was a key clue that helped researchers determine the age of the system.
Using evolutionary models, the team deduced that the outer star formed about 3-5 billion years ago. Since the three stars (one of which is now a black hole) were probably born together, that makes the entire system, black hole and all, also the same age. They also found that the black hole consumed at least half the solar mass of matter from its nearest star.
This discovery has great potential for understanding the formation and evolution of stars, black holes and nearby companions. “This system is extremely interesting for the evolution of black holes and also raises the question of whether there are more triple holes out there,” Burdge said.
