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Illustration showing the accretion disc around a black hole, in which the inner region of the disc is shaken. | Credit: NASA
Astronomers have observed a star wobble in its orbit around a supermassive black hole that is tearing it apart and fizzing with its stellar material. The observation is evidence of a rare and elusive phenomenon called “Lense-Thirring precession” or “frame dragging”, in which a rapidly rotating black hole drags the very fabric of space and time with its motion.
This turn of space-time they first emerged from Albert Einstein‘s 1915 theory of General relativitywhich predicted that objects with mass “twist” the fabric of space and time (united as a single entity called spacetime) and that gravity results from this geometric effect. The greater the mass of the object, the greater its impact on space-time and therefore the greater its gravitational influence. In 1918, the concept of massive, rotating objects dragging space-time along with it 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, which means that the new research could offer scientists a new way to study the spin of black holesthe way in which they eat, or “accrete”, matter torn from stars in tidal disruption events (TDE), and how TDEs give rise to powerful outflows, or jets.
“Our study shows the most convincing evidence yet of Lense-Thirring precession — a black hole that drags spacetime along with it in the same way that a spinning top can drag water around it in a whirlpool,” said team member Cosimo Inserra of Cardiff University in the United Kingdom, in a statement. “This is a real gift for physicists as we confirm a prediction made more than a century ago. Not only this, 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.”
See the wobble
The team began investigating Lense-Thirring precession by studying the TDE designated AT2020afhd using X-ray data collected by NASA’s Swift Neil Gehrels Observatory (Swift), and radio wave observations from the Earth-based Karl G. Jansky Very Large Array (VLA).
A TDE occurs when a star wanders 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 within the star that squeeze it horizontally while simultaneously pulling it vertically. This process, called spaghettification, creates a line of stellar pasta that twists around the black hole like a noodle around a fork, forming a flat cloud called an accretion disk.
Matter from the accretion disk is gradually fed towards the black hole, but these galaxy-dominating titans are notoriously messy eaters, with some material driven from the black hole’s poles by strong magnetic fields. From there, this matter is blasted out as twin plasma jets of almost light speed.
Both the accretion disk of these black holes that perform the TDE and the jets that come out radiate strongly across the electromagnetic spectrum, and because these emissions originate from immediately outside the black hole, they must be affected by Lense-Thirring precession. This effect translates to a “wobble” 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 coming from this TDE which implied that the accretion disk and the jet were shaking with this unisoning motion, every day.
“Unlike previous TDEs studied, which have stable radio signals, the signal for AT2020afhd showed short-term changes, which we could not attribute to the release of energy from the black hole and its surrounding components,” continued Inserra. “This further confirmed the drag effect in our minds and offers scientists a new method to probe black holes.”
Modeling the data from Swift and the VLA, the team was able to confirm that these variations were the result of frame-dragging. Further analysis of these results may help scientists better understand the physics behind the Lense-Thirring effect.
“By showing that a black hole can drag time in space and create this effect that drags the frame, we are also starting to understand the mechanisms of the process,” said Inserra. “Therefore, in the same way a charged object creates a magnetic field when it rotates, we are seeing how a massive rotating object – in this case a black hole – generates a gravitomagnetic field that influences the movement of stars and other cosmic objects nearby.
“It is a reminder to us, especially during the festive season as we look up at the night sky in wonder, that we have in our hands the opportunity to identify more and more extraordinary objects in all the variations and flavors that nature has produced.”
The team’s research was published on Wednesday (December 10) in the journal Science Advances.