More than four years ago, a supermassive black hole at a center of a galaxy 300 million light-years away from Earth devoured a passing star, releasing a burst of X-ray activity.

Researchers from MIT analyzed all available data on the rare sight from multiple telescopes and found the intense and stable signal that appears to emanate from an area very close to the supermassive black hole's event horizon — the region in which the gravitational pull is so intense that nothing, not even light, can escape.

They detailed their discovery in the journal Science.

Signal From A Dying Star

The researchers do not know what exactly caused the signal, but they were able to use it to study the supermassive black hole. While fairly common in the universe (there is one dormant giant at the center of the Milky Way), black holes remain a mystery. Because not even light can escape its pull, black holes are pretty much invisible except when it is feeding on stars.

"Events where black holes shred stars that come too close to them could help us map out the spins of several supermassive black holes that are dormant and otherwise hidden at the centers of galaxies," said Dheeraj Pasham, the first author of the study.

So, to learn more about the supermassive black hole, the researchers took advantage of the rare event. They followed the signal that they said pulsed every 131 seconds and persisted for at least 450 days.

From its stable proximity to the black hole and the black hole's mass, the team of researchers was able to calculate the speed in which the black hole is spinning: about 50 percent the speed of light. They claim that this is the first time that a tidal disruption flare was used to estimate the black hole spin.

A Way To Study The Spin Of A Black Hole

Theoretically, when a black hole shreds a star apart, not everything gets pulled into the darkness. Some will stay outside of the event horizon, orbiting in an area called the Innermost Stable Circular Orbit or ISCO (smallest orbit an object can travel around a black hole) and giving off X-rays before eventually falling into the center.

The length of time that the X-ray flashes before it disappears could give scientists an idea of the size of the black hole's ISCO and give an estimate of how fast the black hole is spinning.

The researchers note that the event would be rare and would only last for about a few hundred years — an extremely short amount of time in cosmic scale. However, it is a system that works and the researchers hope to use it to study the spins of other black holes in the future.