The Nature paper says that the precession rate was about 3 Hz. In other words, the periapsis would circle around about 3 times per second. For comparison, the

The Nature paper says that the precession rate was about 3 Hz. In other words, the periapsis would circle around about 3 times per second. For comparison, the system with the next fastest precession is a binary pulsar system that has a precession rate of ~10^-10 Hz. So this is 10 orders of magnitude faster. This was determined by analyzing the gravitational radiation waveform from the merger. Properties of the system like the masses, spins, and their orientations all leave unique signatures in the gravitational radiation waveform. You can take a look at the original paper here (it's not very long): https://www.nature.com/articles/s41586-022-05212-z https://www.nature.com/articles/s41586-022-05212-z