A garden hose blasts a powerful stream of water. If you pinch it or give it a jerk, the stream starts to writhe like a snake, and spray flies everywhere. Something similar happens with the giant plasma jets that black holes shoot out. These jets are twisted by magnetic fields and can lose stability, starting to wobble.

Scientists studied a stellar-mass black hole — an object several times heavier than the Sun, compressed to the size of a city. Radio telescopes picked up bursts of radiation repeating at an almost constant rhythm — such oscillations are called quasi-periodic. At the same time, not only the brightness of the radio waves changed, but also their polarization (the direction of oscillation of the electromagnetic wave). Brightness and polarization behaved like a seesaw: when one goes up, the other goes down.

A computer model showed that all these oddities are explained by a kink instability — a wave that runs along the jet and makes it sway. Such a wave arises when the magnetic field inside the jet is twisted too tightly, like an overtwisted rubber band. Because of it, the jet compresses in some places and expands in others, changing both brightness and polarization. This discovery provides a new tool: by observing the rhythm of flickering, we can peer into the very heart of a black hole and understand how its magnetic field is structured.