🌀 A star reveals spacetime's drag near a black hole

Astronomers have captured an unusual ballet: a star, torn apart by the titanic forces of a supermassive black hole, exhibits rhythmic oscillations in its orbit that do not match the classical models of tidal disruption events (see below).

These periodic motions originate from a subtle effect of general relativity. When a massive object spins rapidly, it drags the spacetime around it, much like a rotating propeller would create a whirlpool in water. This interaction, known as Lense-Thirring precession, had been theorized but rarely detected with such clarity.


To investigate this phenomenon, the study focused on a specific event, designated AT2020afhd, by combining X-ray and radio wave data. The researchers noticed regular fluctuations in the emissions, with a cycle repeating every twenty Earth days. These modulations synchronized between the accretion disk and the plasma jets allowed confirmation of the spacetime drag hypothesis.

These results offer a new method for studying the spin of black holes and their behavior during the consumption of stellar material. Cosimo Inserra, from Cardiff University, indicated that these observations constituted a significant advance in testing the predictions of general relativity. They also enrich our understanding of the mechanisms at work during the destruction of stars.

Furthermore, the observed phenomenon can be compared to the generation of a gravitomagnetic field by a rotating massive object. In the same way that a moving electric charge produces a magnetic field, a spinning black hole influences the motion of nearby celestial bodies.

The detection of this type of oscillation in a tidal disruption event had until now been rare. The radio signals, usually stable, here exhibited short-term fluctuations, ruling out other explanations related to energy release. This work is published in Science Advances.

Supermassive black holes and their influence

At the center of most galaxies, including our own, reside supermassive black holes. These objects concentrate a mass equivalent to millions, even billions, of times that of the Sun in a reduced volume. Their gravitational attraction is so intense that nothing, not even light, can escape once past the event horizon.

The presence of such a giant profoundly affects its galactic environment. It can distort the orbits of nearby stars, accelerate matter to extreme speeds, and emit intense radiation. These processes play an important role in the evolution of galaxies, influencing star formation and the distribution of matter.

The spin of a supermassive black hole adds an additional dimension to its impact. Like a massive object in motion, it can drag spacetime around it, altering the trajectory of orbiting matter. This effect, although weak, becomes measurable under specific conditions, such as during violent interactions with a star.

Understanding these mechanisms helps to explain observable phenomena, such as plasma jets emitted at speeds close to that of light. These structures, often symmetrical, originate from the polar regions of the black hole and extend across intergalactic distances, transporting energy through the cosmos.

Tidal disruption events

When a star comes too close to a supermassive black hole, it experiences extreme tidal forces. These forces stretch the star in the direction of the attraction while compressing it laterally, a process often called 'spaghettification'. The star is thus deformed and partially dislocated.

The stellar debris then forms an accretion disk around the black hole. In this structure, matter swirls at high speed, heating up through friction and emitting intense radiation across various wavelengths. This luminous phase allows astronomers to detect and study these events from Earth.

Some of the matter from the disk is gradually drawn towards the black hole, crossing the event horizon. Another fraction can be ejected as collimated jets, propelled by powerful magnetic fields. These emissions offer clues about the physical conditions reigning near the black hole.

The study of these events provides information about the density, composition, and dynamics of the stars involved. It also allows testing of general relativity predictions in intense gravitational fields, where the classical effects of Newton's laws are no longer sufficient to describe the observations.