When two black holes merge, space shudders with gravitational waves, and the newborn hole receives a powerful kick, like a cannon recoiling. In the centers of galaxies, it often slams into a gas disk — a swirling cloud of matter, like a cosmic bullseye.

A direct hit punches clean through the disk, briefly extinguishing the jets of searing plasma but heating the gas to a flare. A glancing blow makes the hole ricochet: it dives into the disk and pops out again, creating a series of pulsating bursts. These scenarios differ in their radiation signatures, from radio waves to X-rays, which spectroscopy studies.

The first calculations came from Einstein and Schwarzschild. Modern models with magnetic fields show the kick can be so strong that the hole is ejected from the galaxy forever. Detectors, with contributions from Weiss, will help spot these wandering black holes.