On March 23, an international research team led by Yuan Feng, a researcher at the Shanghai Astronomical Observatory of the Chinese Academy of Sciences and a professor at the Center for Astrophysics and Gravitation, Department of Physics, Fudan University, used the jet of the supermassive black hole in galaxy M87 to study the correctness of the two leading black hole jet models. The results were published in Science Advances.
The team calculated radiation predicted by the two models and compared it to observational data. They found that the jet predicted by the model that extracts energy from the black hole's rotation via magnetic fields aligned very well with actual observations. On the other hand, the model that extracts energy from the rotating accretion disk via magnetic fields had difficulty explaining the observational data. They found that high-energy electrons in the radiation-emitting jet are accelerated by a process called "magnetic reconnection." Magnetic reconnection occurs because the accretion disk around black holes can produce "magnetic bursts" that strongly disturb the magnetic field, resulting in magnetic reconnection in the jet.
How do jets form?
It is well known that due to powerful gravity, nothing, not even light, can escape from within a black hole's event horizon. However, as early as 1918, astronomers discovered through observation that just outside a black hole's event horizon, they can eject powerful outflows containing mass and energy outward at near the speed of light—these are known as jets. Telescope images show that jets shoot outward in a straight line like a laser beam to great distances, with some even extending well beyond galactic scales.
How do these mysterious jets form? This problem has been studied for over 100 years, with many scholars contributing, including Nobel laureate in physics Roger Penrose. Currently, there are two main models in the field. One model extracts the rotational energy of the black hole from large-scale magnetic fields ("black hole rotation energy extraction model"). The other model also requires large-scale magnetic fields, but in this case magnetic fields extract rotational energy from the accretion disk ("accretion disk rotation energy extraction model").
"Neither of these models addresses a very critical question, namely whether the jets predicted by the model are consistent with observational results of jet morphology, width, velocity fields, polarization, and so on," says Yuan Feng.
To answer this question, the Yuan Feng-led international research team used the Shanghai Supercomputer Center and supercomputers at the Shanghai Jiao Tong University-based C. N. Yang Institute for Theoretical Physics to carry out calculations. Through rigorous quantitative calculations, they obtained predicted observational results for the jet and compared them to observational data on jets. They found that "magnetic reconnection" in the black hole jet leads to the acceleration of high-energy electrons emitting radiation in the jet.
New tools for analyzing black hole jets
On April 10, 2019, the first image of a black hole was released. It was of the supermassive black hole in galaxy M87. The research team used this black hole as an object of study to further explore characteristics of black hole jets.
The team used a large-scale numerical simulation method to solve general relativistic magnetohydrodynamic equations and obtain the black hole accretion flow and jets produced by the two models. The team assumed that electron acceleration occurs through the "magnetic reconnection" mechanism in the jet, and combined the physical mechanism of magnetic reconnection accelerated electrons with the results of research on particle acceleration obtained using kinetic theory. By solving a steady-state electron energy distribution equation, they obtained the electron energy spectrum and density in different spatial regions of the jet.
On this basis, within the general relativistic framework, the team calculated radiative transfer in the jet to obtain various predicted observational results, and compared them with real observational data. The results showed that the jet predicted by the "black hole rotation energy extraction model" is consistent with the morphology of observed jets, and the jet's "edge brightening," width, length, and velocity fields are also consistent with observational data.
A comparison between the predicted jet morphology from the "extracting black hole rotational energy" model and that from real observations. Image credit: Science Advances
Yuan said, "This work has bridged the gap between observational data on black hole jets and dynamical models, helping astronomers better utilize telescope data to probe the physical nature of the jets."
Related paper information: http://doi.org/10.1126/sciadv.adn3544
