Physics Spotlight  
This artist's impression shows hot gas orbiting in a disk around a rapidly-spinning black hole. The elongated spot depicts an X-ray-bright region in the disk, which allows the spin of the black hole to be estimated. This artist's impression shows hot gas orbiting in a disk around a rapidly-spinning black hole. The elongated spot depicts an X-ray-bright region in the disk, which allows the spin of the black hole to be estimated.
Image: NASA/CXC/M. Weiss

X-ray pulse detected near event horizon as black hole devours star

Pulse pattern suggests distant black hole must be spinning at least at 50 percent the speed of light.

Jennifer Chu | MIT News Office
January 9, 2019

On Nov. 22, 2014, astronomers spotted a rare event in the night sky: A supermassive black hole at the center of a galaxy, nearly 300 million light-years from Earth, ripping apart a passing star. The event, known as a tidal disruption flare, for the black hole’s massive tidal pull that tears a star apart, created a burst of X-ray activity near the center of the galaxy. Since then, a host of observatories have trained their sights on the event, in hopes of learning more about how black holes feed.

Now researchers at MIT and elsewhere have pored through data from multiple telescopes’ observations of the event, and discovered a curiously intense, stable, and periodic pulse, or signal, of X-rays, across all datasets. The signal appears to emanate from an area very close to the black hole’s event horizon — the point beyond which material is swallowed inescapably by the black hole. The signal appears to periodically brighten and fade every 131 seconds, and persists over at least 450 days.

The researchers believe that whatever is emitting the periodic signal must be orbiting the black hole, just outside the event horizon, near the Innermost Stable Circular Orbit, or ISCO — the smallest orbit in which a particle can safely travel around a black hole. [Full article]