Take three black holes and throw them into the disk surrounding a supermassive black hole and things get really weird, really fast.
That’s the conclusion of new research digging into a particularly strange gravitational wave event that scientists observed in May 2019 and are still trying to understand. Gravitational waves are the ripples in space-time caused by, among other dramatic events, the mergers of black holes.
But this particular observation didn’t match other collisions scientists have caught: it resulted in a black hole in the mid-size range that scientists can barely see, much less explain, and some force was stretching the typically circular dance as the behemoths approached each other.
The black holes, which share the name PKS 2131-021, are locked in a dance of doom about 9 billion light-years from Earth, according to a study published Feb. 23 in The Astrophysical Journal Letters. The two objects have moved steadily toward each other for about 100 million years, according to a statement from NASA, and now they share a binary orbit, with the two black holes orbiting each other every two years or so.
“The gravitational wave event GW190521 is the most surprising discovery to date. The black holes’ masses and spins were already surprising, but even more surprising was that they appeared not to have a circular orbit leading up to the merger,” Imre Bartos, a physicist at the University of Florida and co-author on the new research, said in a statement. (Astronomers name gravitational wave signals with the date they were observed, so GW190521 marks a gravitational wave detected on May 21, 2019.)
PKS 2131-021 is a special type of black hole known as a blazar — basically, a supermassive black hole that happens to be pointing a jet of supercharged matter directly at Earth. That matter originates from the rings of hot gas that form around certain black holes; when a black hole draws in that gas with its powerful gravity, some matter may escape, instead being propelled away in a jet of plasma traveling at nearly the speed of light.
In the earliest analysis of the strange signal, scientists had already suspected that the merger occurred in a pocket of space rich with black holes. Astronomers know of two types of black holes.
The authors of the new study were monitoring the brightness of about 1,800 blazars scattered around the universe when they noticed something peculiar: The brightness of blazar PKS 2131-021 fluctuated at regular intervals — so predictably, in fact, that the study authors likened the fluctuations to the ticking of a clock.
One, dubbed stellar black holes, form from dying stars and contain perhaps a dozen times the mass of our sun. Supermassive black holes, in contrast, hide at the center of some galaxies (including our Milky Way) and can contain millions of times the mass of their puny counterparts.
The result of the May 2019 merger appeared to be an intermediate black hole, a size category that ranges from perhaps 100 to 1,000 times the mass of our sun. Scientists had never yet managed to study and could not explain how such an object might form.
In response to the detection, astronomers suggested that one of the colliding black holes was itself the result of a collision, pushing the final product into the mysterious intermediate range at 142 times the mass of the sun.
To get two sequential collisions, the astronomers first analyzing GW190521 proposed that the event occurred near what scientists call an active galactic nucleus — a particularly dynamic supermassive black hole anchoring a galaxy, where smaller black holes might proliferate.
So physicists behind the new research set about modeling black hole collisions. And while their calculations suggested that three black holes at random were unlikely to result in an eccentric collision, something changed when they considered the environment of an active galactic nucleus.
This type of feature also hosts a disk of matter surrounding the supermassive black hole, like a much more massive model of the solar system. Where the solar system has planets, an active galactic nucleus has stellar black holes scattered throughout the disk in what the new research identifies as essentially a two-dimensional system.
Under those conditions, the probability of an eccentric merger in the models shot up — as much as 100 times, co-author Johan Samsing, an astrophysicist at the Niels Bohr Institute in Denmark, said in the statement. At that rate, perhaps half of mergers in the disks of active galactic nuclei would be eccentric rather than circular, making the exceptional observation of May 2019 much less surprising.
The researchers noted that the probability of eccentric mergers in their model varies with characteristics of the disk surrounding the c. Next up, they said, is spotting ever more black hole collisions to analyze.