An international research team, including scientists from DTU Space, has made a rare observation that provides exceptional insight into the extreme environment just outside a black hole.
The observations reveal a pattern in the light from a disrupted star that follows Einstein’s predictions for how spacetime is curved and dragged around by a rotating black hole.mThe relativistic effect observed is known as the Lense–Thirring effect, a direct consequence of Einstein’s theory of general relativity.
“This is very direct, observation-based evidence for relativity, close to a black hole,” says Senior Researcher Giorgos Leloudas from DTU Space, co-author of the scientific paper reporting the discovery and just published in Science Advances.
A torn-apart star in a very distant galaxy
The event observed by the researchers occurred in the distant galaxy LEDA 145386, about 120 million light-years from Earth. Here, a star passed so close to a supermassive black hole that it was shredded by its enormous gravitational forces - an event known as a Tidal Disruption Event (TDE).
When a star is torn apart, some of its material is flung outward, while the rest spirals toward the black hole, forming a glowing ring of gas known as an accretion disk. At the same time, powerful beams of energy and matter, called jets, are launched from the region near the black hole at speeds close to that of light.
Such events are difficult to study in detail because they occur so far away.
But as the electromagnetic radiation from the event in the form of X-rays and radio waves reached Earth, it was detected by multiple telescopes in space and on the ground. These included NASA’s NICER X-ray telescope on the International Space Station, for which DTU Space provided the equipment enabling precise pointing, as well as the Very Large Array (VLA) radio telescope network in the United States.
Blinking almost like a cosmic lighthouse
Together, these instruments made it possible to measure tiny fluctuations in the light from the disrupted stellar material. The light varied periodically, almost as if it were blinking in a regular rhythm, a bit like a lighthouse.
According to Einstein’s general theory of relativity, spacetime is curved around massive objects such as black holes. If a black hole rotates, it not only curves spacetime but also drags it around with it. This phenomenon is the so-called Lense-Thirring effect.
As a consequence, both the accretion disk and the nearby jet gradually change orientation. Seen from Earth, this appears as periodic variations in the observed light. This is exactly the signature the researchers found. It provides a directly observable confirmation that spacetime near this black hole is strongly curved, just as Einstein’s theory predicts.
“If the black hole is spinning, it will ‘drag’ spacetime along with it and cause both the jet and the donut-shaped disk to change orientation - much like the sweeping beam of a lighthouse. This produces the periodic changes in light that we observe,” explains Giorgos Leloudas.
The X-ray and radio data show synchronized periodicity, leading the researchers to conclude that the disk and the jet are changing orientation in a synchronous way, which is called co-precessing. This is rarely observed, and it reinforces the researchers’ assumptions.
“Direct observational evidence of disk-jet co-precession remains elusive. Here, we report the most compelling case to date,” says lead author of the Sciences Advances article, Yanan Wang, from the National Astronomical Observatories of China.
An important step in understanding black holes
The theory of general relativity is essential for describing phenomena in strong gravitational fields and at velocities close to the speed of light, as is the case around a black hole. Newton’s laws do not work in such environments, where time and space behave very differently because gravity affects the very fabric of spacetime.
Previous observations have hinted at similar behaviour, but the new measurements provide a robust contribution to our understanding of the phenomenon.
The fact that the researchers can measure exactly the kind of light variations predicted by theory offers strong support for general relativity. And for our current models of black holes, that are among the most mysterious objects in the cosmos.
“These observations bring us closer to understanding what is actually happening around black holes. At the same time, we confirm that Einstein’s general relativity still holds. It is remarkable and impressive that his equations from 1915 continue to withstand every test,” concludes Giorgos Leloudas.
