The advent of gravitational wave detectors — there are now four of them — has recorded a steady stream of black hole mergers. As far as we can see, almost all of them have behaved exactly as we would expect for the kind of events we predicted would emerge: a pair of orbiting black holes gradually spiraling inward until they meet in their mutual center of gravity .
But there was one event that apparently didn’t quite match the kind of signals we’d expect. And researchers now suggest it was the product of something that should be incredibly rare: two black holes colliding in the vastness of space. After a single short pass, the two bodies turned a corner and immediately collided.
Templates and beeps
Black hole collisions require the two black holes to be close enough to gravitationally interact. Since space is so vast, this would typically mean 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 toward each other and radiate energy in the form of gravitational waves as they do.
This leads to a relatively straightforward inspiration and fusion, the details of which have appeared in countless animations in the wake of LIGO’s first detection of a black hole collision.
This kind of collision is so well worked out that we have a large number of simulations that model a collision like this with different details: different black hole masses, different spins, and so on. These simulations provide “templates” of the last few moments before the collisions, when gravitational wave production becomes both faster and more intense, with the final “chirp” of waves rising above background noise on Earth. These templates allow us to quickly identify the details of a collision based on how closely the signals from the collision match one of these templates.
But a merger named GW190521 didn’t really fit the templates very well and only matched best if the black holes involved weren’t spinning at all. The chirping 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 of this size do not form in supernovae (which usually start at less than 15 solar masses), so these are probably the product of previous collisions. Which makes it a questionable proposition to start them as part of a binary system.
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 mutual orbit, but just happened to get close enough to gravitationally bond.
Shall we Dance?
The technical term for what the authors propose is “dynamic capture,” which explains the apparently sudden, burst-like nature of the GW190521 signal. Instead of the gradual approach with gravitational waves increasing in intensity typical of binary systems, the two bodies that caused this event could experience a limited number of rapid swings past each other before impact.
The researchers modeled several possible approaches, some of which would lead to a gradual approach similar to those in binary systems and others that could send both black holes away from each other on altered trajectories. But between the two extremes is a range of results where you can either have a small number of short passes before colliding, or the two black holes can collapse directly into each other.
The models that produced a beep that best matched the GW190521 signal saw a single pass that pulled the black holes closer, followed by a single fast turn in the collision. But the first pass was so far away that the signal would be too low to be above the background noise in the detectors. While it is possible to produce similar results with a more typical impact profile with a gradual inspiration, several statistical tests suggest that the dynamic uptake is more likely.
That is probably at least based on the properties of the gravitational waves. The probability of two black holes getting close enough to trigger the process is another matter. But these two black holes are so massive that they were probably created by previous mergers, suggesting that this collision happened in a dense star cluster where many massive stars are dying. So the environment may be more conducive to a chance encounter than we might expect.
Nature Astronomy2022. DOI: 10.1038/s41550-022-01813-w (About DOIs).