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

Astronomers from MIT, NASA and elsewhere have found a new way to measure how fast a black hole is spinning, using the wobbly aftermath of the star party.

The method uses a tidal disruption in a black hole: a blazingly bright moment when a black hole exerts tides on a passing star, tearing it to shreds. As the star is disrupted by the black hole’s enormous tidal forces, half of the star is blown away while the other half is flung 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 created accretion disk is key to working out the inherent spin of the central black hole.

In a study appearing in Naturethe astronomers report that they have measured the spin of a nearby supermassive black hole by tracking the pattern of X-ray flashes that the black hole produced immediately after a tidal disruption.

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

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

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

“By studying several systems with this method over the coming years, astronomers can estimate the overall distribution of black hole spins and understand the long-standing question of how they evolve over time,” said Kavli member Pasham MIT’s Institute for Astrophysics. 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 elsewhere.

Shredded heat

Every black hole has an inherent spin that has been shaped by its cosmic encounters over time. For example, if a black hole has grown largely through accretion – brief moments when some material falls onto the disk, this causes the black hole to spin at quite high speeds. On the other hand, if a black hole grows mainly by merging with other black holes, any merger could slow things down as the spin of one black hole encounters the spin of the other black hole.

As a black hole spins, it drags the surrounding space-time with it. This drag effect is an example of the 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 the surrounding space and time. Normally, this effect would not be clearly visible around black holes, because the massive objects do not emit light.

But in recent years, physicists have proposed that in cases such as during a tidal disruption event, or TDE, scientists might have a chance to track the light from stellar debris as it is dragged around. Next, they might hope to measure the black hole’s spin.

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

When the disk encounters the black hole’s spin, it wobbles as the black hole pulls it into alignment. Eventually, the wobble decreases 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 spin.

“But the key was to have the right observations,” says Pasham. ‘The only way you can do this is that once a tidal disruption happens, you need a telescope that can look at this object continuously for a very long time, so you can investigate all kinds of time scales, from minutes to minutes. to months.”

A catch with a high cadence

For the past five years, Pasham has been looking for tidal disturbances bright enough and close enough to quickly monitor and detect signs of Lense-Thirring precession. In February 2020, he and his colleagues were lucky to detect AT2020ocn, a bright flash coming from a galaxy about a billion light-years away, which was initially spotted in the optical band by the Zwicky Transient Facility.

The optical data showed that the flash was the first moments after a TDE. Because it was both bright and relatively close, Pasham suspected that the TDE could be an ideal candidate to look for signs of wobbly disks, and possibly measure the spin of the black hole at the center of the host galaxy. But for that he would need much more data.

“We needed fast, high-rhythm data,” says Pasham. “The key was to catch this early, because this precession, or wobble, should only be present early. Later, the disk should no longer wobble.”

The team found that NASA’s NICER telescope could 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-rays around black holes and other extreme gravitational objects.

Pasham and his colleagues studied NICER’s observations of AT2020ocn for 200 days after the tidal disruption was first detected. They found that the event emitted X-rays that appeared to peak every 15 days for several cycles before eventually disappearing.

They interpreted the spikes as moments when TDE’s accretion disk wobbled face-first, emitting X-rays directly at NICER’s telescope, before wobbling away while continuing to emit and wave away from him). ).

The researchers took this pattern of wobble and incorporated it into the original theory for the Lense-Thirring precession. Based on estimates of the mass of the black hole and that of the disrupted star, they were able to estimate 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 disk after a tidal disruption 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 determine black hole spins.

“The rotation of a supermassive black hole tells you about the history of that black hole,” says Pasham. ‘Even if a small fraction of those Rubin catches 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 covering news about MIT research, innovation and education.