Strange black hole merger may have been a rare random encounter

A distorted star field with two black objects in the center.
Enlarge / Simulation of two black holes on the verge of a collision.

The advent of gravitational wave detectors (there are now four) has recorded a steady stream of black hole mergers. As far as we can tell, almost all of them have behaved exactly as we would expect for the kind of events we had predicted would produce them: a pair of orbiting black holes that gradually rotate inward until they meet at their mutual center of gravity. .

But there was one event that apparently didn’t coincide with the type of signals we would expect. And researchers now suggest that it was the product of something that should be incredibly rare: two black holes meeting in the vastness of space. After a single close step, the two bodies curved and immediately collided.

Jigs and Squeaks

Black hole collisions require the two black holes to be close enough to each other to interact gravitationally. Since space is so vast, this would normally 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 would slowly spiral closer, radiating energy in the form of gravitational waves as they do so.

This leads to a relatively simple inspiration and fusion, the details of which have appeared in countless animations following LIGO first detection from a collision of black holes.

Collisions of this type are so well resolved that we have a large set of simulations that model a collision like this with different sets of details: different black hole masses, different spins, etc. These simulations provide “templates” of the final moments before collisions, when the production of gravitational waves becomes faster and more intense, with the final “chirp” of waves rising above the background noise on Earth. These templates allow us to quickly identify the details of a collision, based on how closely the collision signals match one of these templates.

But a merger called GW190521 didn’t really fit the templates particularly well and only fit better if the black holes involved weren’t spinning at all. The chirp was unusually short and there is no sign of a signal before the actual meltdown. Finally, the two objects involved in the merger were relatively massive: around 50 and 80 times the mass of the Sun. Black holes of this size do not form in supernovae (which typically start with less than 15 solar masses), so which are likely to be products of previous collisions. Which makes starting as part of a binary a questionable proposition.

So a team of European researchers decided to model an event that should be relatively rare: The two black holes didn’t start out in mutual orbit, but passed close enough to gravitationally lock each other.

We Dance?

The technical term for what the authors propose is “dynamic capture,” which explains the seemingly sudden, burst-like nature of signal GW190521. Instead of the gradual approach in which gravitational waves build in intensity that characterizes binary systems, the two bodies that triggered this event could experience a limited number of high-speed oscillations with each other before a collision.

The researchers modeled a variety of potential approaches, some of which would lead to a gradual approach similar to that seen in binary systems, and others that could push both black holes away on altered trajectories. But in between the two extremes is a set of results where you could have a small number of close passes before the collision, or the two black holes could plunge right into each other.

The models that produced a chirp that best matched the GW190521 signal saw a single step that brought the black holes closer, followed by a single fast curve toward collision. But the first pass was far enough that the signal was too low to rise above the background noise in the detectors. While it is possible to produce results similar to this using a more typical collision profile with gradual inspiration, various statistical tests suggest that dynamic capture is more likely.

That’s probably based on the properties of the gravitational wave chirp, at least. The probability that two black holes will get close enough to trigger the process is an entirely different matter. But these two black holes are massive enough that they were likely built by previous mergers, suggesting that this collision took place in a dense cluster where many massive stars are dying. So the environment may be more favorable for a chance meeting than we might expect.

nature astronomy2022. DOI: 10.1038/s41550-022-01813-en (About DOIs).

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