In the centers of distant galaxies, X-ray 'beacons' sometimes light up. They don't flash randomly but on a strict schedule: every few hours or days. These bursts are called quasi-periodic eruptions (repeating at nearly regular intervals).
The most plausible scenario is this: around a supermassive black hole orbits a gas disk left over from a disrupted star. Through it, like a bullet, a small companion, a star or a stellar-mass compact black hole, tears. Twice per orbit it crashes into the disk, and the impact produces a bright X-ray flash. Such a system is a natural laboratory for studying the extreme close approach of massive bodies, known as gravitational-wave inspiral with extreme mass ratio.
The authors calculated how often such pairs are born. They got about three stellar mergers per million galaxies per year and roughly six for black holes. Then they estimated the contribution of these systems to the stochastic gravitational-wave background (a faint hum from many distant events). It turned out that only black holes can 'sound' in the 1–10 millihertz range accessible to LISA. Their signal could even exceed the antenna's sensitivity threshold by a hundred times. Stars, on the other hand, fall silent at lower frequencies, remaining below the level of audibility.
Quasi-periodic eruptions are such a new phenomenon that the first candidate was only found in 2019, and since then fewer than a dozen such sources have been discovered.
The idea of beacons marking gravitational wells is reminiscent of Orson Scott Card's 'Ender's Game', where signals were used for navigation near massive objects.