Using fluctuating 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 how fast a black hole is spinning, using the wobbly aftermath of its stellar feast.

The method uses a black hole tidal disruption event – ​​a blindingly bright moment when a black hole exerts tidal forces on a passing star, tearing it apart. When a star is disrupted by the massive tidal forces of a black hole, 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 wobbles of the newly formed accretion disk are key to determining the natural rotation of the central black hole.

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

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

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

The study’s lead author, MIT research scientist 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 probe the spins of many nearby black holes, they can begin to understand how gravitational giants have evolved over the course of the universe’s history.

“By studying several systems over 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.

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

Crushed heat

Each black hole has its own spin, which has been shaped by its cosmic encounters over time. For example, if the black hole grew mostly by accretion—brief instances where some material falls onto the disk, causing the black hole to spin at relatively high speeds. In contrast, if a black hole grows mostly by merging with other black holes, each merger could slow things down as the spin of one black hole meets 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 generated by a black hole, can affect the surrounding space and time. Normally, this effect would not be apparent around black holes because massive objects do not emit any light.

But in recent years, physicists have suggested that in cases like a tidal disruption event, or TDE, scientists might have a chance to watch light from stellar debris as it’s dragged around. Then they could hope to measure the spin of the black hole.

Specifically, during a TDE, scientists predict that a star can fall onto a black hole from any direction, creating a disk of white-hot, shredded material that could be tilted or misaligned with respect to the black hole’s rotation. (Think of the accretion disk as a tilted donut spinning around the donut hole, which has its own, independent rotation.)

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

“But the key was to have the right observations,” says Pasham. “The only way to do that is, once there’s a tidal disturbance, you have to get a telescope to look at that object continuously, for a very long time, so you can examine all kinds of time scales from minutes.” for months.”

High cadence catch

For the past five years, Pasham has been searching for tidal disruption events that are bright enough and close enough to quickly track and trace the signs of the Lense-Thirring precession. In February 2020, he and his colleagues were lucky enough to detect AT2020ocn, a bright flare emanating from a galaxy about a billion light-years away that was originally spotted in the optical band by the Zwicky Transient Facility.

From the optical data, the flash appeared to be the first moment after the TDE. Because it is both bright and relatively close, Pasham suspected that TDE might be an ideal candidate for looking for signs of disc wobbles and possibly measuring the rotation of the black hole at the center of the host galaxy. But for that he would need much more data.

“We needed fast data with a high cadence,” says Pasham. “The key was to catch it early, because this precession or wobble should only be present early. Any later and the disk won’t wobble anymore.”

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

Pasham and colleagues examined NICER observations of AT2020ocn during the 200 days following 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 interpreted the peaks as times when the TDE accretion disk wobbled to face itself, emitting X-rays directly towards the NICER telescope, before deflecting as it continued to emit X-rays (much like waving a flashlight towards someone and away from someone every 15 days). .

The researchers took this wobble pattern and incorporated it into the original Lense-Thirring theory of precession. Based on estimates of the mass of the black hole and the mass of the disrupted 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 disk 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, Pasham foresees more opportunities to detect the rotations of black holes.

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

This story is republished courtesy of MIT News (web.mit.edu/newsoffice/), a popular site that covers news about MIT research, innovation, and teaching.