The strangeness of the real universe is hidden in the constellation Cygnus, about 7,800 light-years away from Earth. There, a black hole in a system called V404 Cygni repeatedly performs behavior that both baffles and delights scientists.
Now it’s pulled a whole new trick from its seemingly endless arsenal. It is an invisible binary companion star, orbiting a wide orbit of about 70,000 years.
Since V404 Cygni already has a companion star in a close orbit with a period of 6.5 days, on which the central black hole is leisurely feasting, the newly discovered third object makes the system a triple star system.
This is the first time a system with this configuration has been seen, and it could provide insight into how black holes form. This is because the supernova explosion, which is thought to be the mechanism by which stellar-mass black holes form, should have broken the weak gravitational bond of the expansive orbit.
“We think that most black holes form from violent explosions of stars, but this discovery helps raise questions about this.” Physicist Kevin Burge says: of the Massachusetts Institute of Technology.
“This system is very interesting for black hole evolution, and it also raises the question of whether there are more triples out there.”
In fact, we’ve known about second stars for decades. Astronomers thought it was just a nearby star, the relatively inconspicuous V404 Cygni.
But data collected by the European Space Agency’s Gaia mission shows that there’s definitely more going on than we thought. Gaia is mapping the three-dimensional positions of objects in the galaxy. But as we move through space, we also map direction and speed.
V404 Cygni, a seemingly unrelated star, is moving through space in the same direction and at the same speed. This indicates that the objects are linked.
“It’s almost certainly not a coincidence or an accident.” verge says. “We are seeing two stars that are following each other because they are bound by a weak gravitational string. So this must be a triple system.”
There is evidence for the supernova model of black hole formation. This is when a dying star erupts in a huge explosion, spewing out extraneous material while the star’s core collapses under gravity to form a black hole, the densest object in the universe.
Scientists have inferred the creation of black holes by observing supernovae, disentangling light and measuring the mass of the object at the center. However, this does not mean that supernovae are the only formation mechanism. Another option is the direct collapse model. Here, a massive star completely implodes into a black hole without any disturbance or disturbance.
Evidence is a little more difficult to obtain here. Without confusion or disturbance, there is little evidence.
This is where V404 Cygni suddenly becomes very interesting. That’s because, given that supernova explosions are often asymmetric, the energy imbalance can give directionality to the nascent black hole.
In orbits as wide as newly connected stars, this is difficult to solve. The black hole and star are separated by a distance of 3,500 astronomical units, so their gravitational connection is relatively weak. The chaos caused by the supernova should have broken the link like a spider’s web.
Wide orbital separation also makes gravitational capture between two passing objects difficult to explain. Burge and his colleagues performed tens of thousands of simulations and found that the best explanation was that the three objects were already gravitationally bound when the black hole formed. And the formation mechanism was direct collapse.
“Most simulations show that the easiest way to accomplish this triple task is through direct collapse.” Verge says.
This is the best evidence for the direct collapse model of black hole formation, supporting the mechanism as a valid way to interpret black holes whose formation history as supernovae is difficult to resolve.
There may also be triplets containing other large black holes that we have missed due to the stealth nature of black holes. Finding this could help us better understand how these objects form and why black holes can collapse directly rather than explode in a spark of light.
“Either we were very lucky, or third rounds were common.” Astronomer Kareem El-Badry says: of Caltech.
“If they are common, they could solve some of the long-standing questions about how black hole binaries form. Triplets open up evolutionary pathways not possible in pure binaries.
“People have actually predicted that black hole binaries could mostly form through triple evolution, but there has been no direct evidence until now.”
This study was published in: nature.
