A distorted star field and two black objects in the center.
To enlarge / A simulation of two black holes on the verge of collision.

The advent of gravitational wave detectors—there are now four of them—recorded a steady stream of black hole mergers. As far as we can tell, almost all of them behaved the way we expected them to: a pair of orbiting black holes gradually spiraling inward until they meet at their mutual centers of gravity. .

But something happened that didn’t quite match the types of signals we were expecting. Now researchers suggest it’s the product of something that should be incredibly rare: two black holes meeting each other in the vastness of space. After a single close pass, the two bodies swung around and immediately began to collide.

Templates and jingles

Black hole collisions require two black holes to be close enough to each other to interact gravitationally. Since space is so vast, this would usually mean that they are the product of two massive stars that formed as a binary system. After the stars died and left behind black holes, the two bodies slowly spiraled toward each other, releasing energy as gravitational waves.

This leads to a relatively simple inspiration and combination where the details are visible countless animations After LIGO first detection black hole collision.

Such collisions are so well worked out that we have a large set of simulations that model such a collision with different sets of details: different black hole masses, different spins, etc. These simulations provide “templates” of the last few minutes before the collisions, when the production of gravitational waves is both faster and more intense, with the final “chirp” of the waves rising above the background noise on Earth. These templates allow you to quickly determine the details of a collision based on how closely collision signals match one of these templates.

But the compound, GW190521, didn’t really fit the templates very well and only fit best if the black holes didn’t spin at all. The chirp was unusually short and there is no sign of a signal before the actual merger. Finally, both objects involved in the merger were relatively massive: about 50 and 80 times the mass of the Sun. Black holes of this size do not form in supernovae (they usually start with less than 15 solar masses), so they are likely the product of previous collisions. This makes it dubious to suggest that they started out as part of a binary system.

So a group of European researchers decided to model a relatively unusual phenomenon: two black holes did not start out in mutual orbit, but passed close enough to be held together by gravity.

Shall we dance?

The technical term proposed by the authors is “dynamic capture,” which explains the sudden, burst-like nature of the GW190521 signal. Instead of the gradual approach in which gravitational waves build up to the intensity that characterizes binary systems, the two bodies that cause this event can take a limited number of high-speed swings past each other before colliding.

The researchers modeled a variety of potential approaches, some of which would lead to a gradual approach similar to that seen in binaries, while others would push both black holes apart on altered trajectories. But between the two extremes there is a range of results where you can either get a small number of close passes before a collision, or the two black holes can collapse directly into each other.

The jingle models that best fit the GW190521 signal saw a single transition of approaching black holes, followed by a single fast curve towards the collision. But the first pass was so far away that the signal would be too low to overcome the background noise in the detectors. Although it is possible to obtain similar results using a more typical collision profile with gradual inspiration, various statistical tests indicate that dynamic capture is greater.

This is probably at least based on the properties of gravitational wave chirping. The probability of two black holes getting close enough to trigger the process is an entirely different matter. But the two black holes are massive enough that they were likely built by previous mergers, suggesting that the collision took place in a dense cluster where many massive stars died. Thus, the environment may be more conducive to chance encounters than we might expect.

Natural Astronomy2022. DOI: of 10.1038/s41550-022-01813 (About DOI).

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