When a star wanders too close to a supermassive black hole, tidal forces stretch it and tear it apart in what astronomers call a tidal disruption event. The black hole releases a burst of ultraviolet and X-ray light that clears out its immediate surroundings.
Farther out, a few trillion miles from the black hole, a patchy web of dust survives. It absorbs the flare and re-emits it as infrared light. Using NASA’s Wide-field Infrared Survey Explorer (WISE), a team led by Sjoert van Velzen of Johns Hopkins University found these infrared echoes around three black holes, and researchers at the University of Science and Technology of China found a fourth. It was the first time such echoes had been clearly seen from multiple events.
The delay between the original flare and its echo tells astronomers how much energy was released as the star was destroyed and where the surrounding dust lies.
via Scientists caught black holes swallowing stars - and burping energy back up | The Sydney Morning Herald