Astronomers back in 2014 were searching the night sky for something cool and found what they were looking for. On November 22, 2014, astronomers discovered a supermassive black hole at the center of a galaxy that lies 300 million light-years from Earth. That black hole had captured a passing star and was ripping it apart.
As the star was devoured, an event known as a tidal disruption flare occurred. That even created a burst of X-ray activity at the center of the galaxy and since the discovery, multiple observatories have had telescopes trained on the event to learn more about how precisely black holes consume stars. Researchers at MIT and other locations have been going over the data that the telescopes trained on the black hole have gathered and something interesting was discovered.
An intense, stable, and periodic pulse of X-rays has been found across all datasets. Researchers believe that the signal is emanating from very close to the event horizon of the black hole. The event horizon is the point where material captured by the black hole can no longer escape.
The research found that the signal periodically brightens and fades every 131 seconds and persisted over at least 450 days. The team believes that whatever is emitting the signal has to be orbiting just outside the event horizon in an orbit called the Innermost Stable Circular Orbit or ISCO. That is the smallest orbit that a particle can safely travel around the black hole.
Mass for the black hole is about 1 million times the mass of the sun, and given the stable proximity of the signal to the black hole, the team has calculated that the black hole is spinning at a rate of at least 50% of the speed of light. The team’s results give a way to measure the spin of supermassive black holes using tidal disruption flares. Estimating the spin has been tough to do until now. Researchers say this black hole isn’t particularly fast; others are spinning at 99% the speed of light. The victory here is the ability to use tidal disruption flares to constrain the spin.