Using wobbly stellar material, astronomers measure the spin of a supermassive black hole for the first time

Astronomers at MIT, NASA and elsewhere have a new way to measure a black hole’s spin speed, using the wobbly effects of its stellar feast.

The method exploits a black hole tidal disruption event – the flashy moment when a black hole exerts a tidal wave on a passing star and rips it to pieces. As the star is disrupted by the black hole’s massive tidal forces, half of the star is blown away while the other half is thrown around the black hole, creating an intensely hot accretion disk of rotating stellar material.

The MIT-led team has shown that the wobble of the newly formed accretion disk is key to determining the inherent spin of the central black hole.

In a study that appeared in Natureastronomers report that they have measured the spin of a nearby supermassive black hole by following the pattern of X-ray flashes produced by the black hole just after a tidal disturbance.

The team monitored the flashes for several months and determined that they were likely the signal of a blazing hot accretion disk rocking back and forth as it was pushed and pulled by the spin of the black hole itself.

By tracking how the disk’s wobble changed over time, scientists were able to determine how much the disk was affected by the spin of the black hole, and in turn, how fast the black hole itself was spinning. Their analysis showed that the black hole is spinning at less than 25 percent the speed of light – relatively slowly, as black holes do.

The study’s lead author, MIT researcher Dheeraj “DJ” Pasham, says the new method could be used to measure the spin of hundreds of black holes in the local universe in the coming years. If scientists can probe the spins of many nearby black holes, they can begin to understand how gravitational giants have evolved over the history of the universe.

“By studying several systems in the coming years with this method, astronomers can estimate the overall distribution of black hole spins and understand the long-standing question of how they evolve over time,” says Pasham, who is a member of the Kavli Institute for Astrophysics and MIT. Space research.

Co-authors of the study include collaborators from a range of institutions, including NASA, Masaryk University in the Czech Republic, the University of Leeds, Syracuse University, Tel Aviv University, the Polish Academy of Sciences and elsewhere.

Chopped heat

Each black hole has an inherent spin that is shaped by its cosmic encounters over time. If, for example, the black hole grew mostly through accretion—short bursts of material falling onto the disk, this causes the black hole to spin at a fairly high rate. In contrast, if a black hole grows mainly by merging with other black holes, each merger could slow things down as the spin of one black hole collides with the spin of another.

As a black hole spins, it drags the surrounding spacetime along with it. This drag effect is an example of Lense-Thirring precession, a long-standing theory that describes the ways in which extremely strong gravitational fields, such as those produced by a black hole, can pull on surrounding space and time. Normally, this effect would not be apparent around black holes, since massive objects do not emit light.

But in recent years, physicists have suggested that, in cases such as during a Tidal Disturbance, or TDE, scientists may have a chance to track the light from stellar debris as it drifts around. Then they could hope to measure the spin of the black hole.

Specifically, during a TDE, scientists predict that a star could fall onto a black hole from any direction, generating a disk of glowing, shredded material that could be tilted or misaligned relative to the black hole’s spin. (Think of the accretion disk as a tilted donut spinning around a donut hole that has its own, separate spin.)

As the disk encounters the spin of the black hole, it wobbles as the black hole pulls it into alignment. Eventually, the wobble subsides as the disk settles into the black hole’s spin. Scientists predicted that the disk of the TDE should be a measurable signature of the black hole’s spin.

“But the key was to have the right observations,” says Pasham. “The only way you can do that is, as soon as there is a tidal disturbance, you have to get a telescope that will observe this object continuously, for a very long time, so you can probe all kinds of time scales, from minutes to months.”

Capturing high cadence

For the past five years, Pasham has searched for tidal disturbance events that are bright enough, and close enough, to quickly track and follow the signatures of Lense-Thirring precession. In February 2020, he and his colleagues got lucky, with the detection of AT2020ocn, a bright flash emanating from a galaxy about a billion light-years away that was originally observed in the optical range by the Zwicky Transient Facility.

According to the optical data, the flash appears to have been the first instant after the TDE. Because it is both bright and relatively close, Pasham suspected that the TDE might be an ideal candidate for looking for signs of a wobbly disk and possibly measuring the spin of the black hole at the center of the host galaxy. But for that he would need a lot more data.

“We needed quick and fast data,” says Pasham. “The key was to catch this early because this precession, or wobble, should only be present early on. A little later, and the disk would no longer wobble.”

The team found that NASA’s NICER telescope was able to capture the TDE and keep an eye on it continuously for months. NICER—short for Neutron star Interior Composition ExploreR—is an X-ray telescope on the International Space Station that measures X-ray radiation around black holes and other extreme gravitational objects.

Pasham and his colleagues reviewed NICER observations of AT2020ocn over 200 days after the initial detection of the tidal disruption event. They found that the event emitted X-rays that peaked every 15 days, over several cycles, before eventually fading.

They interpreted the peaks as moments when TDE’s accretion disk swung face-on, emitting X-rays directly at the NICER telescope, before swinging back and continuing to emit X-rays (similar to waving a flashlight to and from someone every 15 days ).

The researchers took this wobble pattern and incorporated it into the original theory for Lense-Thirring precession. Based on estimates of the mass of the black hole and the mass of the perturbed star, they were able to arrive at an estimate of the black hole’s spin – less than 25 percent of the speed of light.

Their results mark the first time scientists have used observations of a wobbly disc after a tidal disturbance to estimate the spin of a black hole. As new telescopes like the Rubin Observatory come online in the coming years, Pasham foresees more opportunities to determine the black hole’s spin.

“The spin of a supermassive black hole tells you the history of that black hole,” says Pasham. “Even if a small fraction of those captured by Rubin 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.”

This story is republished thanks to MIT News (web.mit.edu/newsoffice/), a popular site covering news about MIT research, innovation, and teaching.