Using oscillating stellar material, astronomers have measured the spin of a supermassive black hole for the first time

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Astronomers at MIT, NASA and elsewhere have a new way to measure how fast a black hole is spinning, using the fluctuating effects of its stellar feast.

The method takes advantage of a black hole tidal collapse event—a flash-bright moment when a black hole exerts tidal forces on a passing star and rips it apart. As the star is torn apart by the black hole’s massive tidal forces, half of the star is blown away while the other half is flung around the black hole, generating an intensely hot accretion disk of spinning stellar material.

The MIT-led team showed that the wobble of the newly formed accretion disk is key to developing the intrinsic spin of the central black hole.

In a study published in Natureastronomers report that they have 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 disturbance event.

The team tracked the flashes for several months and determined that they were likely a signal of a blazingly hot accretion disk lurching back and forth as it was pushed and pulled by the black hole’s own rotation.

By tracking how the disk’s oscillations change over time, scientists can determine how much the disk has been affected by the black hole’s rotation and, in turn, how fast the black hole itself is spinning. Their analysis showed that the black hole is rotating at less than 25 percent of the speed of light—relatively slow, like black holes.

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

“By studying several systems over the next few 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 Pascham, who is a member of the Institute for Kavli Astrophysics at MIT and Space Research.

The study’s co-authors include collaborators from a number 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 others.

Grated heat

Each black hole has an intrinsic spin that is shaped by its cosmic encounters over time. If, for example, a black hole has grown primarily through accretion – brief instances where some material falls onto the disk, this causes the black hole to spin up to quite high speeds. In contrast, if a black hole grows mostly by merging with other black holes, each merger can slow things down as the spin of one black hole collides with the spin of the other.

When a black hole rotates, 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 generated by a black hole, can pull on surrounding space and time. Normally, this effect would not be apparent around black holes because massive objects do not emit light.

But in recent years, physicists have suggested that in cases like during a tidal disruption, or TDE, scientists might have a chance to track light from stellar debris as it drifts around. They may then hope to measure the spin of the black hole.

In particular, during a TDE, scientists predict that a star can fall onto a black hole from any direction, generating a disk of white-glowing, jagged material that can be tilted or misaligned with respect to the black hole’s rotation. (Think of the accretion disk as a tilted donut rotating around a donut hole that has its own, separate spin.)

When the disc encounters the spin of the black hole, it wobbles as the black hole pulls it into alignment. Eventually, the wobble subsides as the disc settles into the black hole’s rotation. The scientists predicted that the wobbly disk of the TDE should therefore be a measurable signature of the black hole’s rotation.

“But the key was having the right observations,” says Pascham. “The only way you can do that is as soon as a tidal disruption occurs, you have to get a telescope to look at that object continuously, for a very long time, so you can study all kinds of time scales, from minutes to months.” “

High cadence catch

For the past five years, Pasham has been looking for tidal disturbance events that are bright enough and close enough to quickly track and trace for signs of Lense-Thirring precession. In February 2020, he and his colleagues lucked out with the discovery of AT2020ocn, a bright flash emanating from a galaxy about a billion light-years away that was first spotted in the optical band by the Zwicky Transient Facility.

From the optical data, the flash appears to have been the first moments after the TDE. Because it is both bright and relatively close, Pascham suspects that TDE may be the perfect candidate to look for signs of wobble in the disk and possibly measure the spin of the black hole at the center of the host galaxy. But for that he will need a lot more data.

“We needed fast, high-cadence data,” says Pascham. “The key was to catch this early because this precession or oscillation should only be present early. Later, and the disk will no longer wobble.’

The team discovered that NASA’s NICER telescope was able to capture the TDE and continuously observe it for months. NICER – short for Neutron star Interior Composition ExploreR – is an X-ray telescope on the International Space Station that measures the X-ray emission around black holes and other objects of extreme gravity.

Pascham and colleagues reviewed NICER observations of AT2020ocn for 200 days after the initial detection of the tidal disruption event. They found that the event emitted X-rays that appeared to peak every 15 days, for several cycles, before eventually dying out.

They interpret the peaks as times when the TDE accretion disk swings face-on, emitting X-rays directly at the NICER telescope, before swinging while continuing to emit X-rays (similar to waving a flashlight to and from someone every 15 days ).

The researchers took this wobble pattern and worked it into the original Lense-Thirring precession theory. Based on estimates of the mass of the black hole and that of the collapsed star, they were able to come up with an estimate of the black hole’s rotation – 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 disruption event to estimate the rotation of a black hole. As new telescopes like the Rubin Observatory come online in the coming years, Paschamp foresees more opportunities to determine the spins of black holes.

“The rotation of a supermassive black hole tells you about the history of that black hole,” says Pascham. “Even if a small fraction of those that Rubin picks up 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. “

More info:
Dheeraj Pasham, Lense–Thirring precession after a supermassive black hole destroys a star, Nature (2024). DOI: 10.1038/s41586-024-07433-w.

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