On March 23, an international team led by Yuan Feng, a researcher at the Shanghai Astronomical Observatory of the Chinese Academy of Sciences and now a professor at the Center for Astrophysics and Gravitation of the Department of Physics at Fudan University, studied the jets of the supermassive black hole at the center of the M87 galaxy to investigate the correctness of the two main black hole jet models. Their findings were published in the journal Science Advances.
The team calculated the radiation predicted by the two models and compared it with observational results. They found that the jets predicted by the model that extracts the black hole's rotational energy through magnetic fields were very consistent with actual observational results, while the model that extracts the rotational energy of the black hole's accretion disk through magnetic fields had trouble explaining the observational results. They found that the high-energy electrons in the jets that emit radiation were accelerated by the "magnetic reconnection" process, and magnetic reconnection was caused by the "magnetic eruption" that occurs in the black hole's accretion disk. This eruption can cause strong disturbances to the magnetic field, causing magnetic reconnection in the jets.
How are jets formed?
As we know, due to their extremely strong gravity, not even light can escape the gravitational pull within the radius of a black hole. However, as early as 1918, astronomers discovered through observation that just outside the radius of the black hole, the black hole could eject a powerful outflow of matter and energy outward at speeds close to the speed of light - a jet. Images taken by telescopes show that jets are ejected outward in a straight line like a laser beam over great distances. Some jets can even be much longer than the scale of the galaxy.
How are these mysterious jets formed? This question has been studied for over 100 years, and many scholars, including Nobel laureate in physics Roger Penrose, have conducted research on the topic. Currently, there are two main models in this field of study: one is to extract the rotational energy of the black hole from a large-scale magnetic field, which is known as the "black hole rotational energy extraction" model; the other model also requires a large-scale magnetic field, but the magnetic field extracts the rotational energy of the accretion disk, which is known as the "extraction of accretion disk rotational energy" model.
"Both of these models fail to address a very critical issue, which is whether the jets predicted by the model are consistent with observational results regarding the morphology, width, velocity field, polarization, etc. of the jets," Yuan said.
To answer this question, the international team led by Yuan used the Shanghai Supercomputer Center and the supercomputer at the Shanghai Jiao Tong University's Institute for Advanced Physics to perform calculations, and through rigorous quantitative calculations obtained the predicted observational results of the jets. They then compared these results to observational data on jets, and found that "magnetic reconnection" in the black hole jets led to the acceleration of high-energy electrons that produce radiation in the jets.
A new tool for analyzing black hole jets
On April 10, 2019, the first image of a black hole was released, revealing the supermassive black hole at the center of the M87 galaxy. Taking this black hole as the research object, the research team further explored the characteristics of black hole jets.
Using a large-scale numerical simulation method to solve the general relativistic magnetohydrodynamic equations, the team obtained the black hole accretion flow, as well as the jets produced by the two models. The team assumed that electron acceleration occurs through the "magnetic reconnection" mechanism in the jets, and then combined the physical mechanism of magnetic reconnection to accelerate electrons and the results of particle acceleration obtained using kinetic theory to solve a steady-state electron energy distribution equation. This allowed them to obtain the energy spectrum and number density of electrons in different spaces of the jets.
On this basis, the team calculated the radiative transfer in the jets in a general relativistic framework, obtained various predicted observational results, and compared these results with actual observational results. The results showed that the morphology of the jets predicted by the "black hole rotational energy extraction" model was consistent with the morphology of the jets actually observed. In addition, the "edge brightening" of the jets, the width, length, and velocity field of the jets were also consistent with the observational results.
Comparison between the morphology of jets predicted by the model of "extracting black hole rotational energy" and the morphology of jets observed in practice The image is from Science Advances
Yuan Feng introduced: "This work provides a bridge between the observational data of black hole jets and dynamic models, helping the astronomy community to better use telescope observation data to explore the physical nature of the jets."
Relevant paper information: http://doi.org/10.1126/sciadv.adn3544
