Black holes can also be overwhelming! Chinese and foreign scientists reveal the formation process of magnetic trapping disks around black holes

Black holes also have moments of overeating

Chinese and foreign scientists reveal the formation process of magnetic trapping disks around black holes

Can black holes freely and arbitrarily devour matter? On September 1st, the team of Associate Professor You Bei and his collaborators from the Department of Astronomy at the School of Physical Science and Technology of Wuhan University published the latest research results on black holes in the international academic journal "Science", revealing for the first time the magnetic field transport process in black hole accretion flows and the formation process of magnetic trapping disks, indicating that as the devouring of matter continues, black holes will reach a state of "feeding and supporting".

The research work was jointly completed by Wuhan University, Zhejiang University, Chinese Academy of Sciences Shanghai Observatory, Chinese Academy of Sciences Institute of High Energy Physics, Nanjing University, University of Science and Technology of China, Strasbourg Observatory in France, Poland Center for Theoretical Physics and other units. You Bei is one of the corresponding authors of this research paper.

The difference in time between two types of radiation from the same black hole reaching Earth is 8 days

Black holes are one of the most mysterious celestial bodies in the universe, which can be formed through the death of certain stars, such as the collapse of massive stars. It has an extremely strong gravitational field and can capture any material, including light, close to it, making it impossible to directly observe the black hole itself.

In 2019, the Event Horizon Telescope collaborative organization released the first black hole photo in human history. The black hole and its surrounding material form a cute state resembling a red "donut", leaving a deep impression on people.

In physics, the process of a black hole capturing matter is called "accretion", and the captured matter is called "accretion flow". During the process of devouring a black hole, its two poles will also have powerful jets shooting into space, known as relativistic jets. The viscosity of the accretion process can partially convert gravitational energy into radiation energy, resulting in multi band radiation observed by ground and space telescopes. Therefore, through accretion radiation, black holes indirectly reveal their existence. Observing these radiations has also become an important way to study black holes.

Currently, scientists have detected the existence of stellar black holes in many binary systems. This type of binary system is mostly in a stationary state, occasionally entering an explosive state that lasts for months or years, producing bright X-rays. Therefore, it is often referred to as a black hole X-ray binary.

As early as 2018, the Youbei team had noticed a massive eruption of the black hole X-ray binary MAXI J1820+070. By analyzing the observation data of China's first X-ray astronomical satellite, "Huiyan", the You Bei team discovered a coronal mass escaping the strong gravitational field of a black hole and moving outward at high speed, and for the first time observed the velocity evolution of the coronal mass in a black hole binary. This achievement was published in the international academic journal Nature Communications in 2021, with You Bei as the first author.

At a subsequent academic conference, You Bei was inspired by multi band radiation observations of black hole bursts. Analyzing the observation data of the black hole X-ray binary MAXI J1820+070 again, the Youbei team unexpectedly discovered an unprecedented long time delay phenomenon: the radio radiation of the black hole jet lags behind the hard X-ray of the high-temperature gas in the accretion flow by about 8 days.

Based on past experience, both X-ray radiation and jet radio radiation come from near black holes, and their arrival at Earth should be synchronous or with only a very short time delay. The discovery of the phenomenon of long-term delay is a significant breakthrough in the field of astrophysics at the observational level.

The donut is getting bigger

Why is there an 8-day delay?

The Youbei team proposed that the weak magnetic field outside the accretion disk is enhanced by the thermal accretion flow around the black hole, and the larger the radial scale of the accretion flow, the more significant the magnetic field enhancement. At the beginning, the thermal accretion flow in the inner region had not yet formed, the magnetic field was weak, and the jet was almost undetectable. As the hard X-ray reaches its peak, a thermal accretion flow has formed and rapidly expands to transport a magnetic field towards the inner region. The inner region magnetic field strengthens, and a jet has been generated. When the radio radiation reaches its peak, the magnetic field intensity in the inner region reaches the level of the magnetic trapping disk, and the radio radiation of the jet also reaches its peak.

During these 8 days, a very strong magnetic field will rapidly accumulate around the black hole. "This proves that the crown is expanding, which is actually the process by which the 'donut' becomes larger, and there is a 'magnetic amplification effect'," explained You Bei.

In black hole X-ray binary stars, companion matter is captured by the gravitational pull of the black hole and gradually rotates towards the black hole, forming an accretion disk structure that emits light. Before falling into a black hole, accretion disk material releases the gravitational potential energy of the black hole, causing it to be heated. When the heated substance cannot release energy through physical processes, the temperature increases accordingly. High temperature substances will eventually be ionized, forming a high-temperature plasma, also known as a corona. The more black holes consume in the current period, the lower the material supply and the cooling effect, resulting in the expansion of the corona.

When matter spirals into a black hole, it includes radial velocity and circumferential velocity. The radial velocity is very slow, dominated by the circumferential velocity, so the efficiency of material dragging magnetic field lines into the black hole is not high. "At this point, the corona is equivalent to a highway." You Bei explained that the speed at which matter drags magnetic field lines in the corona increases, causing the magnetic field to rapidly strengthen. Therefore, as the corona expands, the "highway" widens, causing the magnetic field near the black hole to rapidly increase and the outward magnetic force on matter to increase.

"Gluttony" overeating

According to theory, as the accretion gas continuously brings in a weak external magnetic field, the outward magnetic force of the magnetic field on the accretion flow gradually strengthens. When the outward magnetic force of the magnetic field is equivalent to the inward gravitational force of the black hole, the accretion material is trapped by the magnetic field and cannot freely and quickly fall into the black hole's visual interface, forming a magnetic trapping disk.

The magnetic confinement disk is a mature theoretical model in the international academic community, widely applied to various celestial bodies, and successfully explains many complex observation phenomena in black hole accretion systems. However, the academic community has always lacked direct evidence of the existence of magnetic confinement disks. How it was formed is still an unsolved mystery.

Based on the phenomenon of long timescale delay, the research team has conducted more than two years of research and found through analyzing X-ray observation data that hard X-ray radiation decreases with decreasing accretion rate, while the radial scale of thermal accretion flow rapidly expands with decreasing accretion rate, causing the magnetic field near the black hole to rapidly increase. About 8 days after the peak of hard X-ray radiation, a magnetic trapping disk is formed.

According to You Bei, black holes were able to "eat freely" in the early days, just like gluttony, but when they reached a certain level, they became "unable to eat". "Our results have proven that gluttony does indeed eat enough and explained how gluttony eats enough." This is the first time in the world that the existence of a magnetic confinement disk has been revealed, and its formation process has been successfully calculated. This achievement has been recognized by international scholars in the review of Science.

It is worth noting that the X-ray observation data provided by China's first space astronomical satellite, the "Huiyan" satellite, played an indispensable role in the discovery of research results. From China's Tianyan FAST and "Huiyan" satellites to the soon to be launched "Einstein Probe" satellites, more and more major weapons with independent intellectual property rights in China are being put into use. You Bei believes that these are very beneficial for Chinese astronomers to complete astrophysical experiments, obtain observational data, and make more original discoveries.

You Bei stated that this achievement will greatly enhance the understanding of key scientific issues such as the formation of large-scale magnetic fields in accretion disks of black holes of different magnitudes and the mechanism of jet acceleration. Next, he hopes to apply this achievement to the study of other black holes in the Milky Way and even the universe, exploring the universality of this physical process and whether it is related to black hole jets.