The advent of gravitational-wave detectors — now there are four of them — have recorded a steady stream of black hole mergers. As far as we can tell, almost all of them behaved exactly as we would expect for the kind of events we predicted would produce: a pair of orbiting black holes that gradually spiral inward, until they meet at their common center of gravity.
But there was one event that didn’t seem to quite match the signals we’d expect. And researchers are now suggesting it was the product of what should be incredibly rare: two black holes found in the vast reaches of space. After a single close pass, the two bodies flexed and immediately collided.
templates and chips
Black hole collisions require the two black holes to be close enough to interact gravitationally. Because space is so vast, this would normally mean that they are the product of two massive stars that formed as a binary star system. After the stars died, leaving behind black holes, the two bodies would slowly spiral towards each other, radiating energy in the form of gravitational waves.
This results in a relatively straightforward inspiration and fusion, the details of which have surfaced in countless animations following LIGO’s initial discovery of a black hole collision.
Collisions of this kind are so well worked out that we have a large number of simulations modeling such a collision with different details: different masses of black holes, different spins and so on. These simulations provide “templates” for the final moments before collisions, when gravitational-wave generation both accelerates and intensifies, with the final “chirp” of the waves drowning out the background noise on Earth. These templates allow us to quickly identify the details of a collision based on how closely the collision’s signals match one of these templates.
But a merger called GW190521 didn’t really fit the templates very well, fitting best only when the black holes involved weren’t spinning at all. The chirp was unusually short and there is no sign of a signal prior to 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 this size don’t form in supernovae (these typically start out at less than 15 solar masses), so they’re likely the product of earlier collisions. This makes launching them as part of a binary system questionable.
So a team of European researchers decided to model an event that should be relatively unusual: the two black holes didn’t start out in a shared orbit, but happened to pass close enough to gravitationally cling to each other.
Shall we Dance?
The technical term for what the authors propose is “dynamic capture,” which explains the seemingly sudden, bursty nature of the GW190521 signal. Instead of the gradual approach, in which the intensity of gravitational waves increases, that is characteristic of binary systems, the two bodies that triggered this event could experience a finite number of high-speed oscillations past each other before colliding.
The researchers modeled a variety of possible approaches, some of which would result in a stepwise approach similar to that in binary systems, and others that could send both black holes away from each other on altered trajectories. But between the two extremes, there are a number of outcomes where you could either have a small number of close flybys before the collision, or the two black holes could crash straight into each other.
The models that produced a chirp that best matched the GW190521 signal saw a single pass that drew the black holes closer, followed by a single fast turn into the collision. But the first pass was so far away that the signal was too weak to stand out against the background noise in the detectors. While it is possible to obtain similar results using a more typical gradual inspiration collision profile, various statistical tests suggest that dynamic capture is more likely.
That’s probably based at least on the properties of the gravitational-wave chirp. The likelihood of two black holes getting close enough to trigger the process is another matter entirely. But these two black holes are massive enough that they likely formed from previous mergers, suggesting this collision occurred in a dense cluster where many massive stars are dying. As such, the setting for a chance encounter may be more favorable than we might expect.
natural astronomy2022. DOI: 10.1038/s41550-022-01813-w (About DOIs).
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