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A black hole rips apart a star, and we see a dazzling flare. Then, a couple of years later, when everyone has forgotten about it, radio telescopes suddenly pick up a new signal, like an echo from the past. Until now, it was unclear why a tidal disruption event (when the hole stretches the star with its tidal forces) sometimes stays 'quiet' in radio, only to suddenly 'light up'.

The authors of the study propose a solution. A compact object may be orbiting close to the black hole — for example, a dead star the size of a city but with the mass of the Sun. It orbits so closely that its orbit gradually shrinks due to the emission of gravitational waves. When the black hole disrupts a normal star, the remnants form a rotating disk. At first, the disk is small, but over time it spreads out, like a puddle of oil on a hot skillet. After a year or two, it reaches the orbit of that neighbor, and repeated collisions begin. Each impact ejects material outward at speeds of about 20–30 thousand kilometers per second.

The ejected material slams into the gas around the black hole and generates radio waves — the very ones we see with a delay. Interestingly, similar collisions can also cause periodic X-ray flares (so-called quasi-periodic eruptions), but the conditions for bright radio and X-ray pulsations do not always coincide. So one simple idea connects several mysterious phenomena that previously seemed unrelated.

Thus, an ordinary stellar disruption turns into a long-running show where invisible partners play the leading roles. And if one day a radio telescope catches a delayed signal, we might be hearing, for the first time, the echo of a star doomed to fall into the abyss.