This research result provides strong observational evidence for the existence of the M87 black hole spin (Figure 1). Cui Yuzhu, a postdoctoral fellow in Zhijiang Laboratory, is the first author and corresponding author of the paper.
On September 27, Nature magazine released the latest research results of an international scientific research team composed of 45 institutions. By analyzing observational data from 2000 to 2022, it was found that the black hole jet at the center of the M87 galaxy shows a periodic swing, with a swing period of about 11 years and an amplitude of about 10 degrees. This phenomenon is in line with the prediction of Einstein's general theory of relativity that "if the black hole is in a rotating state, it will cause a reference frame drag effect." This research result provides strong observational evidence for the existence of the M87 black hole spin (Figure 1). Cui Yuzhu, a postdoctoral fellow in Zhijiang Laboratory, is the first author and corresponding author of the paper.
Figure 1 Schematic diagram of the tilted accretion disk model. Assume that the spin axis of the black hole is vertically upward, the direction of the jet is almost perpendicular to the surface of the accretion disk, and there is a certain angle between the spin axis of the black hole and the rotation axis of the accretion disk, which is a tilted accretion disk model. The angle between the directions of the angular momentum of the black hole and the accretion disk triggers the precession of the accretion disk and the jet. (Source: Yuzhu Cui et al. 2023, Intouchable Lab@Openverse and Zhijiang Laboratory)
Successfully captured the periodic precession of the jet
On April 10, 2019, astronomers from many places around the world simultaneously released the first photo of a black hole. It is located in the center of the neighboring galaxy M87, 55 million light-years away from the Earth, and has a mass 6.5 billion times greater than the sun. Such supermassive black holes are one of the most mysterious and destructive objects in the universe. Their gravity is huge, and they "eat" a large amount of matter through the accretion disk. At the same time, they "spit" the matter out thousands of light-years away at a high speed close to the speed of light, forming jets.
"A strange straight ray emanated from the center of a hazy spot of light." In 1918, astronomers first observed the jets in M87, which was also the first cosmic jet observed by humans. What is the energy transfer mechanism between supermassive black holes, accretion disks and jets? This question has puzzled physicists and astronomers for more than a century.
Currently, the widely accepted theory among scientists is that the angular momentum of a black hole is the source of energy. One possibility is that if there is a magnetic field near the black hole and the black hole is in a rotating state, an electric field will be generated like a conductor cutting magnetic field lines, thereby accelerating the ionized matter around the black hole, and eventually part of the matter will be ejected carrying huge energy. Among them, the spin of supermassive black holes is a key factor in this theory. However, black hole spin parameters are extremely difficult to measure, and there is still no direct observational evidence whether the black hole is in a rotating state.
In order to study this challenging problem, researchers studied the supermassive black hole and its jets at the center of the M87 galaxy. Using Very Long Baseline Interferometry (VLBI) with ultra-high angular resolution, astronomers have resolved the jet structure very close to the black hole. By analyzing VLBI observation data from 2000 to 2022, scientific researchers successfully captured the periodic precession of the jet in M87 (Figure 2) (Precession : A rotating rigid body is acted upon by an external force, causing its rotation axis to rotate around a certain center rotation).
What force can regularly change the direction of this powerful jet? After extensive analysis, the research team concluded that the answer to the problem may be hidden in the dynamic properties of the accretion disk. Materials with a certain angular momentum will orbit around the black hole and form an accretion disk. Due to the gravity of the black hole, they will continue to approach the black hole until they are irreversibly "sucked" into the black hole. However, the angular momentum of the accretion disk can be affected by a variety of random factors, and it is very likely that it has a certain angle with the black hole's spin axis. However, the super strong gravity of the black hole will have a significant 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, thus triggering accretion. The disk and jets precess periodically.
Figure 2 The merged jet structure of M87 every two years from 2013 to 2018 (observation frequency band is 43 GHz). The corresponding year is displayed in the upper left corner. White arrows indicate the jet position angle in each subfigure. Bottom: Best fit based on images merged on a one-year basis from 2000 to 2022. The green and blue points come from data in the observation frequency bands of 22 GHz and 43 GHz respectively. The red line represents the best fit results according to the precession model. (Source: Yuzhu Cui et al. 2023)
The research team conducted a large number of detailed theoretical investigations and analyzes based on the observation results, and combined with the properties of M87, used supercomputers to conduct the latest numerical simulations. The results of numerical simulations confirm that when there is an angle between the rotation axis of the accretion disk and the spin axis of the black hole, the entire accretion disk will precess due to the drag effect of the reference frame, and the jet will also be affected by the accretion disk. Precession. The detection of the precession of the jet can provide strong observational evidence for the spin of the black hole at the center of M87, bringing new understanding of the properties of supermassive black holes.
Uncovering more mysteries of black holes requires computational help
"We are very happy and lucky to have made this important discovery. In 2017, when I was processing the EAVN data of the M87 galaxy, I saw that the jet structure was obviously in a different direction from the previous structure. This started a six-year period of detailed data Processing, a large number of theoretical article research and countless discussions with collaborators." Cui Yuzhu, the first author and corresponding author of the paper and a postdoctoral fellow in Zhijiang Laboratory, said that due to the relatively large angle between the black hole spin axis and the accretion disk angular momentum, The small precession period exceeds ten years, the accumulation of high-resolution data for more than two periods of 23 years, and the careful analysis of the structure of M87 are all necessary conditions for obtaining this result.
"I am very grateful to the help and support of many collaborators, as well as the valuable opinions of journal editors and reviewers. It is worth mentioning that one of the reviewers of our article is James Moran, a legend in the field of VLBI radio astronomy research." Cui Yuzhu said.
It is reported that this work used many observatories including the East Asia VLBI Network (EAVN), the United States' Very Long Baseline Array (VLBA), South Korea's KVN and Japan's VERA Joint Array (KaVA), and the East Asia to Italy/Russia joint EATING observation network. 170 observational data from an international observing network and more than 20 radio telescopes around the world contributed to this research.
Figure 3 Distribution of telescopes participating in this paper in the East Asian VLBI network (Source: Kazuhiro Hada, Yuzhu Cui et al. 2023)
Dr Motoki Kino of Kogakuin University, Coordinator of the EAVN Scientific Working Group on Active Galactic Nuclei, said: "This is an exciting scientific milestone and thanks to years of joint observations by researchers from 45 institutions around the world, we This scientific mystery was finally revealed. The observational data fit perfectly with the predictions of the precession model, greatly advancing our understanding of black holes and jet systems."
"Based on this work, we predict that there are more black holes at the center of galaxies with similar tilted accretion disk structures, but how to detect more objects with tilted disks faces greater challenges. There are many mysteries that need more Long-term observations and more detailed analysis.” Researcher Shen Zhiqiang of the Shanghai Observatory of the Chinese Academy of Sciences, an important cooperating unit of this achievement, said, “The recently started construction of the Shanghai Observatory’s Shigatse 40-meter radio telescope will further enhance EAVN’s high-resolution millimeter Wave imaging observation capabilities are expected to lead to more astronomical discoveries."
Cui Yuzhu said that the fine structure of the accretion disk and the precise spin value of the M87 supermassive black hole still need to be further studied. This further research relies on the search of a very large number of physical parameters and requires the support of super intelligent computing power.
Currently, Zhijiang Laboratory has built the FAST@ZJLAB intelligent computing astronomy open platform, which brings together 17 intelligent algorithms and built scientific databases such as BlinkVerse "blinkverse.alkaidos.cn" and ChemiVerse in fields such as fast radio bursts and astrochemistry. A stable transmission channel has been established with China Sky Eye FAST, and astronomical big data continues to gather.
"China Sky Eye" FAST chief scientist and Zhejiang Laboratory's chief scientist for astronomical computing Li Gu said that the completion of more and more radio telescopes will bring about explosive growth of observation data, and astronomical research increasingly requires the support of intelligent computing. Zhijiang Laboratory is introducing artificial intelligence, cloud computing and other technologies into astronomical research to improve data processing efficiency and expand the exploration space of physical parameters. It is believed that the deep integration of computing science and radio astronomy will effectively promote the revealing of the nature of mysterious phenomena in the universe such as black holes.