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An illustration showing the accretion disk surrounding a black hole, where the inner region of the disk wobbles. | Credit: NASA
Astronomers have observed a star wobbling in its orbit around a supermassive black hole, which is tearing it apart and feasting on its stellar material. The observation is evidence of a rare and elusive phenomenon called “delayed lens precession” or “frame drag,” in which a rapidly spinning black hole pulls the very fabric of space and time around its motion.
This whirlwind of Spacetime first appeared in Albert Einsteinthe theory of 1915 a general relativitywho predicted that objects with mass “warp” the fabric of space and time (united as a single entity called spacetime) and that gravity arises from this geometric effect. The greater the object’s mass, the greater its impact on spacetime, and therefore the greater the gravitational influence. In 1918, the concept of massive, rotating objects dragging spacetime along with them was then solidified using general relativity by Austrian physicists Josef Lense and Hans Thirring.
Since then, however, this effect has been difficult for scientists to observe, meaning the new research could give scientists a new way to study rotation black holeshow they feed on, or ‘accret’, matter torn from stars in tidal disruption events (TDEs), and how TDEs give rise to strong outflows, or jets.
“Our study shows the most compelling evidence yet for Lense-Thirring precession — a black hole dragging spacetime along with it, in the same way that a table might drag water around it in an eddy,” team member Cosimo Inserra of Cardiff University in the UK said in a statement. “This is a real gift for physicists, as we confirm predictions made more than a century ago. Not only that, but these observations also tell us more about the nature of TDEs – when a star is torn apart by the immense gravitational forces exerted by a black hole.”
Watch the wobble
The team began investigating Lense-Thirring precession by studying TDE-designated AT2020afhd using X-ray data collected by a NASA spacecraft, the Neil Gehrels Swift Observatory (Swift), and radio observations from the Karl G. Jansky Very Large Array (VLA).
A TDE occurs when a star gets too close to a supermassive black hole, and the immense gravitational influence of that cosmic titan, which can be as massive as billions of suns, generates tidal forces inside the star that squeeze it horizontally while simultaneously stretching it vertically. This process, called spaghettification, creates a strand of stellar pasta that twists around the black hole like a noodle around a fork, forming a flattened cloud called an accretion disk.
Matter from the accretion disk is gradually fed into the black hole, but these galaxy-dominant titans are notoriously messy eaters, with some material funneled away from the black hole’s poles by strong magnetic fields. From there, this matter is ejected as twin jets of plasma near the speed of light.
Both the accretion disk of these TDE black holes and the jets they erupt radiate strongly in the electromagnetic spectrum, and since these emissions originate immediately outside the black hole, they should be affected by Lense-Thirring precession. This effect translates into an “orbit” in the orbit of matter in the accretion disk around the supermassive black hole. Indeed, while observing AT2020afhd, the team saw rhythmic changes in both X-rays and radio waves from this TDE, implying that the accretion disk and jet wobbled daily with this motion, repeated every day.
“Unlike previous TDEs studied, which have constant radio signals, the signal for AT2020afhd showed short-term changes that we could not attribute to energy release from the black hole and its surrounding components,” Inserra continued. “This further confirmed the drag effect in our minds and gives scientists a new method for probing black holes.”
By modeling the data from Swift and the VLA, the team was able to confirm that these variations were the result of frame firing. Further analysis of these results could help scientists better understand the physics behind the Lense-Thirring effect.
“By showing that a black hole can pull spacetime and create this frame-pulling effect, we’re also beginning to understand the mechanics of the process,” Inserra said. “So, in the same way that a charged object creates a magnetic field when it spins, we see how a massive rotating object – in this case a black hole – generates a gravitomagnetic field that influences the motion of stars and other nearby cosmic objects.
“It’s a reminder to us, especially during the holidays, as we gaze in wonder at the night sky, that we have the opportunity to identify increasingly extraordinary objects in all the variations and flavors that nature has produced.”
The team’s research was published Wednesday (December 10) in the journal Advances in science.