The gravitational lens allowed the Chandra X-ray telescope to very accurately measure the speed of rotation of a black hole in one of the galaxies in the constellation Pegasus. It turned out that it moves around the axis almost as fast as light, scientists write in the Astrophysical Journal.
Any large mass of matter interacts with light and causes its rays to bend in the same way as ordinary optical lenses do. Scientists call this effect gravitational lensing. In some cases, the curvature of space helps astronomers see ultra-distant objects - the first galaxies in the Universe and their quasar cores - that would be inaccessible for observation from Earth without gravitational "increase".
If two quasars, galaxies or other objects are located almost exactly one behind the other for observers on Earth, an interesting phenomenon arises. Light from a more distant object will split when passing through the first object's gravitational lens. Because of this, we will see not two, but five bright points, four of which will be light "copies" of a more distant object.
This structure is often called the "Einstein's Cross" because of the fact that its existence is predicted by the theory of relativity. Most importantly, this same theory says that each copy of an object will be a "photograph" of a quasar, galaxy, or supernova at different periods of their life due to the fact that their light spent a different amount of time to exit the gravitational lens.
Xinyu Dai of the University of Oklahoma in Norman (USA) and his colleagues used Einstein's crosses to solve a problem that many other astronomers previously thought was impossible - they were able to directly measure the rotation speed of several supermassive black holes.
In the past, such measurements were carried out only indirectly, since the blackest hole, despite its enormous mass, cannot be seen and measured. Dai and his colleagues drew attention to the fact that both the mass and the speed of rotation of a black hole are reflected in how its X-rays look and how large the region where it is born is.
This region is almost as small as the black hole's event horizon itself, making it virtually impossible to see it under normal conditions. On the other hand, "Einstein's crosses" allow you to do this if they are superimposed on each other or on other types of gravitational lenses.
Guided by this idea, astrophysicists studied photographs of the night sky taken by "Chandra" and found five quasars at once, whose light was amplified in a similar way. One of them, Q2237 + 0305, was magnified so successfully that scientists were able to measure the speed of rotation of the black hole with record high accuracy.
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This object, located in the constellation Pegasus at a distance of 8 billion light-years from Earth, moves on its axis with impossibly fast, about 70% of the speed of light. The new estimates turned out to be significantly higher than the predictions obtained indirectly, and they are only 8% less than the maximum value allowed by the theory.
Thanks to such a fast rotation, the Earth or any other objects in the vicinity of this black hole would remain stable and would not fall on it even if they were only 2-3 times more distant from the event horizon than the distance between center Q2237 + 0305 and this imaginary line.
Interestingly, the other four objects had a "normal" rotation speed, which was about half that of Q2237 + 0305. Why this is so, scientists cannot yet say, but they assume that these differences reflect what happened to their galaxies in the distant past.