Star debris can be used to measure the spin of black holes

Astronomers have developed a remarkable method to measure the spin of supermassive black holes. To achieve this feat, the scientists harnessed the chaotic aftermath of black holes devouring stars.

This novel approach sheds light on the elusive properties of black holes and paves the way for future discoveries.

Tidal disruption events

A tidal disruption event (TDE) occurs when a star ventures too close to a black hole, resulting in its violent destruction. The black hole’s immense tidal forces rip the star apart, creating an intensely hot accretion disk from the remaining stellar material.

This accretion disk, composed of the star’s remnants, spins and wobbles under the influence of the black hole’s gravity.

Researchers at MIT, in collaboration with NASA and other institutions, have demonstrated that this wobbling disk is crucial for determining the black hole’s spin.

The study revealed that tracking the X-ray flashes emitted by the disk immediately following a tidal disruption event can provide insights into the black hole’s rotational speed.

Unveiling the black hole’s spin

The research team monitored the X-ray emissions from a nearby supermassive black hole over several months, observing the fluctuations caused by the wobbling accretion disk.

By analyzing how the wobble evolved, the experts inferred the black hole’s spin. Their findings indicated that the black hole was spinning at less than 25 percent the speed of light – a relatively slow rate for black holes.

Study lead author Dheeraj “DJ” Pasham is a member of MIT’s Kavli Institute for Astrophysics and Space Research.

“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,” noted Pasham.

Wobbling accretion disk

Black holes acquire their spin through cosmic encounters over time. Accretion, where material falls onto the black hole’s disk, can significantly increase the spin. Conversely, mergers with other black holes may slow it down as the spins of the merging entities counteract each other.

As a black hole spins, it drags the surrounding space-time, a phenomenon known as Lense-Thirring precession.

During a tidal disruption event, the star’s debris forms a tilted, misaligned accretion disk around the black hole. The disk wobbles as the black hole’s spin pulls it into alignment.

The scientists predicted that this wobbling could serve as a measurable signature of the black hole’s spin. However, obtaining the right observations was crucial.

Hunting for tidal disruption events

Pasham and his team spent five years searching for bright, nearby TDEs suitable for detailed study.

In February 2020, they struck gold with the detection of AT2020ocn, a bright flash from a galaxy about a billion light-years away. This event, initially spotted by the Zwicky Transient Facility in the optical band, showed promise for tracking disk wobbling and measuring the black hole’s spin.

“We needed quick and high-cadence data,” said Pasham. “The key was to catch this early on because this precession, or wobble, should only be present early on. Any later, and the disk would not wobble anymore.”

NASA’s NICER telescope, positioned on the International Space Station, provided the continuous observations that were needed.

NICER, designed to measure X-ray radiation around black holes and other extreme gravitational objects, tracked AT2020ocn for 200 days after the tidal disruption event.

The data revealed X-ray peaks every 15 days, corresponding to the wobbling accretion disk emitting X-rays directly towards the telescope.

Decoding the spin of black holes

The research team incorporated these observations into the Lense-Thirring precession theory, estimating the black hole’s spin based on the mass of the black hole and the disrupted star.

The analysis confirmed that the black hole was spinning at less than 25 percent the speed of light, marking the first time this method has been used to estimate a black hole’s spin.

Looking ahead, Pasham anticipates more opportunities to measure black hole spins as new telescopes, like the Rubin Observatory, come online.

“The spin of a supermassive black hole tells you about the history of that black hole,” Pasham said. “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.”

The study is published in the journal Nature.

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