Using flickering stellar material, astronomers measure the rotation of a supermassive black hole for the first time

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

The method takes advantage of a black hole tidal disruption event – ​​an extremely bright moment when a black hole exerts tides on a passing star and tears it to shreds. As the star is disrupted by the black hole’s immense tidal forces, half of the star is blown away, while the other half is thrown around the black hole, generating an intensely hot accretion disk of rotating stellar material.

The MIT-led team showed that the wobble of the newly created accretion disk is key to determining the inherent rotation of the central black hole.

In a study published in NatureAstronomers report that they measured the rotation of a nearby supermassive black hole by tracking the pattern of X-ray flashes that the black hole produced immediately after a tidal disruption event.

The team tracked the flashes for several months and determined that it was likely a signal from a burning accretion disk that was oscillating back and forth as it was pushed and pulled by the hole’s own rotation black.

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

The study’s lead author, MIT researcher Dheeraj “DJ” Pasham, says the new method could be used to evaluate the spins of hundreds of black holes in the local universe in the coming years. If scientists can study 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 multiple 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 change over time,” says Institute member Pasham. Kavli of Astrophysics and Astrophysics from MIT. Space research.

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

Shredded Heat

Every black hole has an inherent rotation that has been shaped by its cosmic encounters over time. If, for example, a black hole grew primarily through accretion – brief instances of material falling onto the disk, this causes the black hole to spin at fairly high speeds. In contrast, if a black hole grows primarily by merging with other black holes, each merger could slow things down as the rotation of one black hole collides with that of the other.

When a black hole rotates, it pulls the surrounding space-time with it. This drag effect is an example of Lense-Thirring precession, a long-standing theory that describes how extremely strong gravitational fields, such as those generated by a black hole, can exert influence on space and time surroundings. Normally, this effect would not be evident around black holes, because massive objects emit no light.

But in recent years, physicists have proposed that in cases such as during a tidal disruption event, or TDE, scientists might have the ability to track light from stellar debris as it is entrained. Then they could hope to measure the rotation of the black hole.

In particular, during a TDE, scientists predict that a star could fall on a black hole from any direction, generating a disk of jagged, white-hot material that could be tilted or misaligned relative to the rotation of the black hole. (Think of the accretion disk as a tilted donut that rotates around a donut hole that has its own separate rotation.)

When the disk encounters the rotation of the black hole, it wobbles as the black hole aligns it. Eventually, the oscillations subside as the disk settles into the black hole’s rotation. Scientists predicted that a TDE’s wobbly disk should therefore be a measurable signature of the black hole’s rotation.

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

High-speed capture

For the past five years, Pasham has searched for tidal disruption events bright enough and close enough to quickly track and detect signs of Lense-Thirring precession. In February 2020, he and his colleagues lucked out with the detection of AT2020ocn, a bright flash emanating from a galaxy about a billion light years away, initially spotted in the optical band by the Zwicky Transient Facility .

Based on the optical data, the flash appears to be the first instant following a TDE. Being both bright and relatively close, Pasham suspected that the TDE might be the ideal candidate to look for signs of disk wobble and possibly measure the rotation of the black hole at the center of the host galaxy. But for that, it would need a lot more data.

“We needed fast, high-throughput data,” says Pasham. “The key was to detect this early on, because this precession, or oscillation, should only be present at the beginning. Later, the disk would no longer oscillate.”

The team found that NASA’s NICER telescope was able to capture the TDE and monitor 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 colleagues examined observations of AT2020ocn made by NICER more than 200 days after the tidal disruption event was initially detected. They found that the event was emitting X-rays that seemed to peak every 15 days, for several cycles, before finally dying down.

They interpreted the spikes as moments when the TDE accretion disk wobbled head-on, emitting X-rays directly toward the NICER telescope, before flickering as it continued to emit waved a flashlight towards and away from someone every 15 days). ).

The researchers took this oscillation model and integrated it with the original Lense-Thirring theory of precession. Based on estimates of the mass of the black hole and that of the disrupted star, they were able to estimate the rotation of the black hole, which is less than 25% of the speed of light.

Their results mark the first time scientists have used observations of a wobbly disk following a tidal disruption event to estimate the rotation of a black hole. As new telescopes such as the Rubin Observatory come online in the coming years, Pasham foresees more opportunities to determine the rotation of black holes.

“The rotation of a supermassive black hole tells you the story of that black hole,” says Pasham. “Even though a small fraction of those captured by Rubin have this type of signal, we now have a way to measure the spins of hundreds of TDEs. We could then make an important statement about how black holes evolve over time. of the age of the universe.

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