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The black hole will engulf the surrounding material, forming a rotating accretion disk. But the theory also predicts that some of the matter in the accretion disk will be bound by a strong magnetic field and cannot fall into the black hole. Recently, scientists have observed this phenomenon for the first time by using several telescopes such as China's X-ray astronomical satellite "discerning Eye".

(by Chen Qiang / tr. by Robert Taylor)

Recently, scientists have used several telescopes, including China's first X-ray astronomical satellite, "discerning Eye," to observe and study the black hole, revealing for the first time the complete formation process of the "magnetic trapping disk" in the black hole accretion disk, providing an important clue for exploring the mysteries of the accretion disk and jet. The results were published in the top international academic journal Science (Science) on September 1.

The research work was carried out by Wuhan University, Zhejiang University, Shanghai Astronomical Observatory of the Chinese Academy of Sciences, Institute of High Energy Physics of the Chinese Academy of Sciences, Nanjing University, University of Science and Technology of China, Strasbourg Observatory in France, Polish Center for theoretical Physics and other units.

Can the accretion disk of a black hole trap matter? Black hole is one of the most mysterious celestial bodies in the universe. it has a strong gravity and can engulf the matter around it. When matter is attracted to a black hole, it forms a rotating disk around the black hole, called an accretion disk.

There is violent friction between the substances in the accretion disk, which produces a lot of heat, and emits all kinds of electromagnetic radiation, such as visible light and X-rays, and occasionally jets that can produce radio wave radiation. By observing these radiation, we can "see" the black hole and analyze the distribution and motion of matter around it.

In addition, the accretion disk is also affected by the magnetic field. The theory is that the accreted material will continue to bring in a weak external magnetic field, which will cause the magnetic field to be stronger as it gets closer to the inside of the accretion disk. When the magnetic field reaches a certain intensity, it can produce an outward thrust on matter and compete with the gravity of the black hole. In this way, in the region of the accretion disk near the black hole, matter is trapped by a magnetic field and cannot fall into the black hole. This part of the accretion disk is called "magnetic confinement disk" (magnetically arrested disk).

Although the theory of magnetic traps has been proposed for many years and can explain some black hole accretion phenomena, there is no direct observational evidence for the existence of magnetic traps before.

The asynchronism of radiation reveals the formation of magnetic trapping disk recently, scientists used several telescopes such as "discerning Eye", China's first X-ray astronomical satellite, to observe the black hole X-ray binary star MAXI J1820070 in X-ray, optical and radio waves. This is a binary system about 10,000 light-years away, made up of a black hole (about eight times the mass of the sun) and a star. Among them, part of the star's material is sucked away by the black hole and forms an accretion disk.

This kind of binary system is quiet most of the time and emits very little electromagnetic radiation. However, they occasionally enter an explosive phase that lasts for months or years, emitting intense X-rays. Such binary systems, known as black hole X-ray binaries, provide scientists with a rare opportunity to observe the evolution of black hole accretion disks.

By analyzing the observed data, the scientists found a very interesting phenomenon: compared with the time when the ultra-bright X-rays next to the black hole reached the peak time, the radio wave radiation reached the peak time by about eight days. Scientists believe that such a long lag is caused by the formation of a magnetic trapping disk in a black hole accretion disk.

Specifically, when the black hole explosion is coming to an end, due to the decrease of accretion rate, the radial scale of high-temperature accretion matter will expand rapidly, which will make the magnetic field near the black hole increase rapidly. In this way, a strong magnetic field appears near the black hole and jets are gradually generated. Magnetic traps are formed in this area about eight days after the peak of X-ray radiation. At this point, the radio wave radiation of the jet reaches its peak.

In addition, scientists have observed that due to X-ray exposure, more external matter will accelerate inward due to instability and produce visible light sparkle in the outer region of the accretion disk. About 17 days after the peak of X-ray radiation, the peak of visible light radiation occurs.

These observations reveal for the first time the complete process of the formation of the magnetic trapping disk in the black hole accretion disk, and provide an important clue for understanding the formation of the high magnetic field in the accretion disk and the mechanism of jet generation and acceleration. Because the accretion matter of different masses of black holes obeys the same physical law, this research result also helps to promote the understanding of other types of black hole accretion, such as supermassive black holes.

References:

Https://www.science.org/doi/10.1126/science.abo4504

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