A loud quasi-periodic oscillation after a star is disrupted by a massive black hole

The tidal forces close to massive black holes can rip apart stars that come too close to them. As the resulting stellar debris spirals toward the black hole, it heats up and emits x-rays. We report observations of a stable 131-s x-ray quasi-periodic oscillation from the tidal disruption event ASASSN-14li. Assuming the black hole mass indicated by host galaxy scaling relations, this implies that (i) the periodicity originates from close to the event horizon, and (ii) the black hole is rapidly spinning. Our findings demonstrate that tidal disruption events can generate quasi-periodic oscillations which encode information about the physical properties of their black holes.