MATHEMATICAL SCIENCES

Chinese scientists have found that the jet of the black hole of the M87 galaxy swings periodically


Six years ago, a “puzzling” astronomical phenomenon piqued Cui Yuzhu’s curiosity.

She spent nearly one year verifying the observation data and analysis process, and then collaborated with researchers from 45 institutions around the world to analyze the observations of multiple Very Long Baseline Interferometry (VLBI) networks from 2000 to 2022, and found that the black hole jet at the center of the M87 galaxy (hereinafter referred to as the M87 jet) showed periodic oscillations. Its oscillation period is about 11 years, and the amplitude is about 10 degrees.

The study, published September 27 in Nature, linked the dynamics of the M87 jet to the state of the supermassive black hole at the center of the galaxy, providing observational evidence for the existence of the M87 black hole’s spin. The paper was highly praised and recommended for publication by the two reviewers, and the journal editor also expected the work to be launched as soon as possible, and even very rarely personally “shot” to polish the abstract.

Illustration of the tilted accretion disk model. Photo courtesy of interviewee

From doubt, curiosity to happiness

In 2017, when analyzing data from the East Asian Very Long Baseline Interferometry (VLBI) network, doctoral student Cui Yuzhu found that the direction of the M87 jet was somewhat different from the previously known angle.

“The M87 jet is a very famous black hole jet, and its ejection angle of 288 degrees is a familiar property. Cui Yuzhu, the first author and corresponding author of the paper and a postdoctoral fellow in Zhijiang Laboratory, told China Science News, “But the observation data of that year showed that the jet direction was about 5 degrees away from the previous data, which is a relatively obvious deviation.” ”

Is it a problem with the observation telescope, a data processing error, or something else?

This made Cui Yuzhu very confused. After thinking for a while, she became curious again.

“If it’s not a data processing error, there must be some structural change behind it, and there are very interesting physical properties involved.” The more Cui Yuzhu thought about it, the more excited he became, because black holes are the simplest and most direct celestial objects to test Einstein’s general theory of relativity, and people need to fully observe black holes and their surrounding activities to understand more of their physical properties, and find that black holes cause certain structural changes in the surrounding environment or jets, which is very important for further understanding of black holes.

Immediately, Cui Yuzhu checked the observation data of several other telescopes and found that there was no problem with data processing. In the following nearly one year, she used data from other international telescope observation arrays to verify that the M87 jet in 2017 was indeed deflected.

“Then the goal is clear, because when we find such a change, we want to see if this deviation is cyclical or accidental, and whether there are some laws in it.” Cui Yuzhu said.

The combined M87 jet structure was merged every two years from 2013 to 2018. Photo courtesy of interviewee

The mystery of the “big top” change

The supermassive black hole at the center of an active galaxy is one of the most destructive and mysterious objects in the universe. They have a huge gravitational force, “eating” large amounts of matter through the accretion disk, while also “spitting out” the matter at high speed. But what happens to the energy transfer mechanism between supermassive black holes, accretion disks, and jets has puzzled physicists and astronomers for more than a century.

At present, scientists widely accept the theory that black hole angular momentum is the source of energy. One possibility is that if there is a magnetic field near the black hole and the black hole is spinning, it will generate an electric field like a conductor cutting magnetic field lines, which will accelerate the ionization around the black hole and cause some of the matter to be ejected with huge amounts of energy. Among them, the spin of supermassive black holes is the key to the establishment of the theory, but the spin parameters of black holes are extremely difficult to measure, and even whether the black hole is in a rotating state has not yet been directly observed.

The M87 galaxy is a neighboring galaxy 55 million light-years from Earth with a black hole at its center that is 6.5 billion times more massive than the Sun. Astronomers first observed jets in M87 in 1918, the first cosmic jets ever observed, features that make the galaxy M87 the best target for studying the relationship between black holes and jets.

After a lot of analysis, scientists believe that the force that changes the direction of the black hole’s jet may be hidden in the dynamic properties of the accretion disk: matter with a certain angular momentum will move around the black hole and form an accretion disk, and they will continue to approach the black hole until they are “sucked” into the black hole by the gravitational action of the black hole. The super-strong gravitational pull of a black hole can have a huge impact on the surrounding space-time, causing nearby objects to be dragged along the direction of the black hole’s rotation, which is the “reference frame drag effect” predicted by Einstein’s general theory of relativity. This effect in turn causes periodic precession of accretion disks and jets (the phenomenon that the spin body rotates around a center when its spin axis is subjected to an external force).

Cui explained that the accretion disk and jet of the M87 black hole are roughly vertical, and if you think of it as a “big top”, the accretion disk is like a gyro body, and the jet up to 5,000 light-years long is its axis of rotation. The difference is that the fulcrum of gyro motion is below it, while the center of motion of the accretion disk is the black hole.

Based on the analysis of observation results and supercomputer simulation, the research team confirmed that when there is an angle between the rotation axis of the accretion disk and the spin axis of the black hole, the precession of the entire accretion disk will be caused by the drag effect of the reference frame, and the jet will also affect the precession due to the influence of the accretion disk.

The precession law of the jet revealed in this study verifies the “reference frame drag effect” predicted by general relativity, provides strong evidence for the existence of spin in the central black hole of M87, and brings new understanding of the nature of supermassive black holes.

Member photo of the VLBI Cooperation Group in East Asia. Photo courtesy of interviewee

The journey of exploration continues

Due to the small angle between the black hole’s spin axis and the accretion disk angular momentum, and the precession period exceeds 10 years, it is necessary to accumulate high-resolution data of more than two periods in order to carefully analyze the M87 structure. The long observation period and huge amount of data information make this research a model for international cooperation to solve the mysteries of the universe.

The work used data from 170 international observing networks, including the East Asia Very Long Baseline Interferometry Network, the Very Long Baseline Array in the United States, the KVN and the VERA Joint Array (KaVA) in Korea, and the Joint EATING Observing Network from East Asia to Italy/Russia, and more than 20 radio telescopes worldwide contributed to the study.

In the course of the project, the research team has cooperated deeply with many domestic units, and the 65-meter Tianma Telescope of the Shanghai Astronomical Observatory of the Chinese Academy of Sciences and the 26-meter Radio Telescope of Nanshan of the Xinjiang Astronomical Observatory have continued to participate in the observation of the VLBI Observation Array in East Asia since 2017, which has played an important role in improving the observation sensitivity and angular resolution.

“I am very happy and fortunate to have made this important discovery. Cui Yuzhu said, “This is also closely related to the improvement of the sensitivity of our Tianma telescope and the improvement of the angular resolution of the Xinjiang Nanshan telescope.” These observational advances allow us to clearly see more structures and lower radiation levels. ”

Shen Zhiqiang, a researcher at the Shanghai Astronomical Observatory of the Chinese Academy of Sciences, believes that recent scientific discoveries have fully demonstrated the advantages of millimeter-wave VLBI technology in studying supermassive black holes and exploring the mysteries of the universe. “Based on this work, it is predicted that there are more black holes at the center of galaxies with similar tilted accretion disk structures, and how to detect more sources will require long-term observations and detailed analysis.”

“With the development of modern astronomy, especially radio astronomy, we have captured huge and rich cosmic signals through radio telescopes, resulting in massive amounts of data. Further integration of the developed frontiers of computational science and radio astronomy exploration will reveal the nature of mysterious phenomena in the universe, including black holes. Li Jing, a researcher at the National Astronomical Observatory of the Chinese Academy of Sciences and chief scientist of computational astronomy at Zhijiang Laboratory, commented.

“Whether black holes spin has always been a central concern for scientists. Dr. Kazuhiro Hada of the National Astronomical Observatory of Japan said, “This result confirms previous expectations from observations, and after this milestone, the journey of exploration continues, allowing us to uncover more mysteries of the universe step by step.” (Source: Zhang Shuanghu, China Science News)

Related paper information:https://doi.org/10.1038/s41586-023-06479-6



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