Researchers have devised a new way to probe supermassive black holes and their evolution across the universe.
A team of scientists from MIT, NASA, and elsewhere has come up with a way to directly measure black hole spins by following the wobble or quiver of an accretion disk during tidal disruption events. By monitoring the X-ray flashes from the event, researchers have found a nearby supermassive black hole with its spin less than 25 percent the speed of light. This could be the new approach toward the evolutionary history of black holes across the universe.
New Method to Measure Black Hole Spins Astronomers have a new way to measure how fast a black hole spins, by using the wobbly aftermath from its stellar feasting.
A black hole tidal disruption event exploitation is used to great advantage by the method — one moment of blinding brilliance in which a black hole applies its tides to a passing star and rips it asunder. Then, equally bifurcated by the strong tidal forces of the black hole, half of the star is blown away, and half flies around the black hole, creating an accretion disk of very hot, rotating stellar material.
Tracking X-Ray Flashes and Black Hole Spin
With a slew of new observations and techniques, the MIT-led team has shown that the wobble of the newly created accretion disk is the important aspect in working out the central black hole’s inherent spin.
The astronomers report that, for the first time, they have measured the spin of a close supermassive black hole; it was done by tracking the almost periodic pattern of the flashes in X-rays that the black hole produced immediately after a tidal disruption event. Over months, the team tracked the flashes and inferred from them that they likely were bright, hot disk of accretion wobbling back and forth, gettingtidally pushed and pulled by the black hole’s own spin.
From how much this wobble itself changed over time, it was possible for scientists to back out exactly how much the disk was being affected by the black hole’s spin, and thus just how fast the black hole itself was spinning. Analysis showed the black hole was spinning at less than 25 percent light speed — relatively slow, as far as black holes go.
Measuring Black Hole Evolution
The lead author of the study, MIT Research Scientist Dheeraj “DJ” Pasham, says the new method could be used to gauge the spins of hundreds of black holes in the local universe within the next few years. If scientists can survey the spins of many nearby black holes, they can begin to understand how the gravitational giants 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 longstanding question of how they evolve over time,” says Pasham, who is a member of MIT’s Kavli Institute for Astrophysics and Space Research.
The study’s co-authors include scientists from NASA, Masaryk University in the Czech Republic, the University of Leeds, Syracuse University, Tel Aviv University, the Polish Academy of Sciences, and others.
Precession of an Accretion Disk Formed From the Debris of a Disrupted Star Around a Supermassive Black Hole
This schematic figure represents the precession of an accretion disk made from the debris of a disrupted star around a supermassive black hole. On the left panel, the phase of the precession is such that the accretion disk is close to being edge-on in configuration—this corresponds to smaller projected disk area, which causes lower luminosity observed by the vulnerary. One sees mostly the colder, outer parts of the processing disk. The right panel is close to face-on precession phase, when visible disk area is larger and hence the luminosity also increases. Then, the inner, warmer parts of the disk are fully exposed. Credit: Courtesy of Michal Zajacek & Dheeraj Pasham
Insights From Tidal Disruption Events
Every black hole has an inherent spin that has been shaped by its cosmic encounters through time. For instance, if a black hole has grown mostly via accretion—brief episodes wherein some material falls onto the disk, this causes the black hole to spin up to nice high speeds. On the other hand, if it grows mostly through merger events with other black holes, then each such merger could slow things down as one black hole’s spin meets up against that of the other.
The drag that a rotating black hole exerts on the surrounding space-time is called drag. It is an effect that is an example of Lense-Thirring precession, a quite old theory describing how very strong gravitational fields—like those produced by a black hole—might pull on the surrounding space and time. Usually, one would not notice this effect around black holes because these huge objects do not emit light.
Still, over the past few years, physicists have begun to argue that, on the odd occasion—something like a TDE—a group of researchers might just get a chance to follow light from the stellar detritus as it gets pulled around. If this happens, scientists would be able to determine, or at least constrain quite tightly, a black hole’s spin.
Specifically, astronomers predict that in the course of a TDE, a star will be torn apart and pulled onto a black hole from any conceivable direction. This generates a white-hot disk of shredded material that could be tilted, or misaligned, with respect to the black hole’s spin. Now, for an accretion disk, imagine a tilted donut; the hole in the donut has a spin of its own and it is spinning around it. The disk wobbles as it meets the black hole’s spin, getting tugged into alignment. After some time the wobble subsides; the disk falls into spin with the black hole. Scientists predicted that a wobbling disk in a TDE ought therefore to be a measurable signature of the black hole’s spin.
“But the key was to have the right observations,” Pasham says. “The only way you can do this is, as soon as a tidal disruption event goes off, you need to get a telescope to look at this object continuously, for a very long time, so you can probe all kinds of timescales from minutes to months.”
A High-Cadence Catch
For the last five years, Pasham has been monitoring for such tidal disruption events that are bright and close enough to quickly follow up and track for signs of Lense-Thirring precession. It wasn’t until February when he and his colleagues hit the jackpot with the detection of AT2020ocn—a bright flash emanating from a galaxy about a billion light years away, first observed in the optical band by the Zwicky Transient Facility.
From the optical data, the flash seemed to be the first moments following a TDE. It was very bright, and relatively speaking, very close by, so Pasham suspected that the TDE might have been an ideal candidate in which to look not only for signs of disk wobbling but also a possible means of measuring spin at the black hole residing at the host galaxy’s center. But he would need much more data for that.
“We needed quick and high-cadence data,” Pasham says. “The key was to catch this early on because this precession, or wobble, should only be present early on. Any later, and the disk would not wobble anymore.”
The researchers found NASA’s NICER telescope to have captured the TDE TDE – and continued to monitor it for months at a stretch. NICER stands for Neutron star Interior Composition ExploreR, an X-ray International Space Station-based telescope that measures X-ray radiations around black holes and other extreme gravitational objects.
Pasham and his colleagues sifted through all the observations that NICER had taken of AT2020ocn during 200 days after the initial detection of this tidal disruption event. Those observations, they said, showed that the event spit out X-rays that appeared to peak every 15 days, several times in succession, before ultimately fading away. They interpreted the peaks as times when the TDE’s accretion disk wobbled face-on, beaming X-rays directly at NICER’s telescope, before wobbling away as it continued to emit X-rays; the net effect was that of waving a flashlight toward and away from someone every 15 days.
Conclusion and Future Prospects
The researchers took this pattern of wobbling and worked it into the original theory for Lense-Thirring precession. With estimates of the mass of the black hole and that of the disrupted star, they could come up with an estimate for the spin of the black hole: less than 25 percent the speed of light.
Their results are the first to date for which scientists have used observations related to a wobbling disk following a tidal disruption event to make an estimate of the spin for a black hole.
“Black holes are fascinating objects and flows of material we see falling onto them can generate some of the most luminous events in the universe,” says co-author Chris Nixon, associate professor of theoretical physics at the University of Leeds. “While there is a lot we still do not understand, there are amazing observational facilities which keep surprising us and generating new avenues to explore. This event is one of those surprises.”
With next-generation telescopes like the Rubin Observatory coming online in the next few years, Pasham says, he expects many more opportunities to pin down black hole spins.
“The spin of a supermassive black hole tells you about the history of that black hole,” Pasham says. “Even if a small fraction of those that Rubin captures 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.”
Reference: “Lense–Thirring precession after a supermassive black hole disrupts a star” by Dheeraj R. Pasham, Michal Zajaček, C. J. Nixon, Eric R. Coughlin, Marzena Śniegowska, Agnieszka Janiuk, Bożena Czerny, Thomas Wevers, Muryel Guolo, Yukta Ajay and Michael Loewenstein, 22 May 2024, Nature.