The recoil imparted as two black holes collide has now been measured using gravitational waves.
It’s the first-ever measurement to capture not just the velocity at which the newly formed black hole was punted across space, but also the direction, offering a new tool for understanding black hole mergers.
From the 2019 gravitational wave event GW190412, astronomers have determined that the lopsidedness of the collision kicked the black hole at speeds exceeding 50 kilometers (31 miles) per second.
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“This is one of the few phenomena in astrophysics where we’re not just detecting something – we’re reconstructing the full 3D motion of an object that’s billions of light-years away, using only ripples in spacetime,” says astrophysicist Koustav Chandra of Pennsylvania State University.
“It’s a remarkable demonstration of what gravitational waves can do.”
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It’s been 10 years since the first detection of gravitational waves, and since that time, the LIGO, Virgo, and KAGRA detectors have caught hundreds of black hole collisions ringing through the Universe.
Gravitational waves are like ripples in a pond, if that pond is spacetime. As two black holes spiral towards each other, their interacting gravitational fields perturb spacetime, sending out ripples at the speed of light.
This dance culminates in one massive gravitational bloop as the black holes collide and merge, forming a single object. Scientists can decode these ripples to probe the properties of the black holes, including the mass and spin of each of the two colliding black holes, as well as the mass of the final merged product.
“Black-hole mergers can be understood as a superposition of different signals, just like the music of an orchestra consistent with the combination of music played by many different instruments,” explains astrophysicist Juan Calderon-Bustillo of the University of Santiago de Compostela in Spain.
“However, this orchestra is special: audiences located in different positions around it will record different combinations of instruments, which allows them to understand where exactly they are around it.”
One of the most dramatic outcomes of a violent cosmic event, such as a core-collapse supernova or a black hole merger, is a phenomenon known as a natal kick. If the event is lopsided – the supernova is more powerful on one side, or the masses of the two black holes are wildly uneven – the energy imparted will be uneven, giving the newly formed black hole a giant shove in one direction.
Back in 2018, Calderon-Bustillo and his colleagues devised a method for measuring the natal kick of a black hole from gravitational wave merger data, based on the spins and masses of the black holes involved. It required a specific set of conditions that had not yet been met at the time, but it didn’t take long for the right type of event to happen.
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In April 2019, a black hole collision between two black holes in a wildly uneven binary was finally detected by the LIGO-Virgo collaboration. One of the black holes clocked in at 29.7 times the mass of the Sun, while the other was over three times smaller – just 8.4 solar masses. Moreover, the light weight of the merger meant a much longer signal than more massive mergers, which presented a wealth of data.
Using their analysis technique, the researchers determined the angle and velocity at which the merged black hole was ejected from its collision – fast enough to be expelled from a globular cluster, a tightly bound cluster of stars within a galaxy.
We don’t know, of course, if the black hole was in a globular cluster; the merger took place 2.4 billion light-years away, and our instruments aren’t high-resolution enough to see a globular cluster that far away. But if it was, it’s probably on its way out.
This technique, the researchers say, could be a powerful new tool for probing black hole mergers.
“Black-hole mergers in dense environments can lead to detectable electromagnetic signals – known as flares – as the remnant black hole traverses a dense environment like an active galactic nucleus,” says astrophysicist Samson Leong of the Chinese University of Hong Kong.
“Because the visibility of the flare depends on the recoil’s orientation relative to Earth, measuring the recoils will allow us to distinguish between a true gravitational wave-electromagnetic signal pair that comes from a binary black hole and just a random coincidence.”
The research has been published in Nature Astronomy.