Most supermassive black holes in the universe are gentle giants, quietly sleeping at the center of galaxies. But if a star comes near, it can suddenly go into a feeding frenzy, ripping it apart and emitting bursts of energy.

This is the case for Swift J1644+57, a supermassive black hole a few million times the mass of the Sun. Its strong tidal forces have shredded a passing star, forming an accretion disk around it that's emitting X-rays. The observation of Swift J1644+57 by US astronomers is reported in Nature. 

“Most tidal disruption events don’t emit much in the high-energy X-ray band. But there have been at least three known events that have, and this is the first and only such event that has been caught at its peak,” lead author Erin Kara, a Hubble Postdoctoral Fellow in astronomy at UMD and the Joint Space-Science Institute, said in a statement.

More surprisingly, the event has been observed in X-rays that have bounced from the black hole and echoed around the accretion disk, amplifying it, similar to how a reflective shield increases the brightness of a flashlight.

Seeing X-rays being formed so close to the black hole is a surprising find. Previous observations have suggested that the most intense electromagnetic radiation, which power the jets of active black holes, happen further out.

“Before this result, there was no clear evidence that we were seeing into the innermost regions of the accretion disk,” Kara said. “We thought the emission was from the jet pointed at us, or further away and not close to the central black hole. This new study shows us that, actually, we can see this reverberation at work very close to the central black hole.”

The most studied supermassive black holes are very active, with powerful jets and hot accretion disks. This could create a bias in understanding these objects as it is believed that 90 percent of supermassive black holes are dormant. This observation gives a unique opportunity to study the quieter phases of these objects. 

The team used the reverberation of X-rays around the accretion disk, like a submarine sonar, to learn about the material distribution. They believe that, in the near future, studying the reflected emission will tell us how quickly the black hole is spinning and how long it took to go back into a dormant state, providing a new approach to better understand dormant supermassive black holes.