Astronomers clock a spinning black hole

Black hole based on artist's drawing (courtesy by NASA)

X-rays blasting from a supermassive black hole feasting on an unlucky star allowed on MIT-led team of researchers to measure how fast the black hole spins – clocking it at nearly 50 per cent the speed of light. It’s the first time a star’s death was used to time a supermassive black hole’s spin.

In 2014, astronomers saw the central black hole inside a galaxy 300 million light-years away lure in and rip apart a passing star. Several telescopes captured this event’s X-ray radiation. More recently, a team combed through the data and noticed regular bursts of X-rays from near the black hole’s event horizon (the point of no return, where material disappears into the black hole) once every 131 seconds. These periodic emissions persisted at least 450 days after the star’s death, boosting the black hole’s total X-ray emissions by 40 per cent.

The team thinks these X-rays were created when material from the star interacted with a white dwarf, the remnant of a Sun-like star, orbiting dangerously close to the black hole at a point called the innermost stable circular orbit a black hole without being devoured. The hot gas from the star illuminated the white dwarf, causing an X-ray glow that appeared each time the white dwarf completed an orbit, every 131 seconds. That allowed the team to deduce how fast the black hole itself is spinning. The team now hopes to track down similar X-ray emissions, shedding more light on how black holes evolve.