Supermassive black hole spin measured for the first time

For the first time, astronomers have been able to determine how fast a black hole is spinning.

The findings provide a new way to study black holes and their evolution.

The new method, detailed in a paper published in Nature, involves observations of the “wobbling” stellar material left over from the black hole’s consumption.

Black hole tidal disruption events (TDEs) are the extremely bright moments when a black hole’s immense gravitational pull exerts tidal forces on stars, ripping them apart. The result is that a portion of the star is blown away, while the rest forms a hot accretion disc of rotating stellar material that orbits the black hole.

A team of astronomers has shown that the “wobble” of this disc is the key to determining the black hole’s spin rate.

The black hole they studied is at the centre of the otherwise quiet galaxy SDSS J135353.80+535949.7 about a billion light-years from Earth. The authors estimate the black hole has a mass about 700,000 times that of our Sun.

In 2020, NASA’s NICER (Neutron star Interior Composition ExploreR mission) instrument aboard the International Space Station analysed an X-ray flash at this galaxy, named AT2020ocn. The astronomers realised that this X-ray flash was produced immediately after a tidal disruption event after tracking flashes for several months.

They measured how the bright, hot accretion disc wobbled as it was pushed and pulled by the spinning black hole.

Analysis showed the black hole spinning at less than 25% the speed of light – relatively slow for a black hole. The new method for measuring black hole spin could help determine the spin rates for hundreds of nearby black holes.

“By studying several systems in the coming years with this method, astronomers can estimate the overall distribution of black hole spins and understand the longstanding question of how they evolve over time,” says lead author Dheeraj “DJ” Pasham from MIT.

It is the first time that the wobble of an accretion disc has been used to determine the speed of a black hole’s spin.

Pasham says new instruments – such as the Rubin Observatory currently under construction in Chile – will help with such measurements.

“The spin of a supermassive black hole tells you about the history of that black hole,” Pasham adds. “Even if a small fraction of those that Rubin captures have this kind of signal, we now have a way to measure the spins of hundreds of TDEs. Then we could make a big statement about how black holes evolve over the age of the universe.”

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