China's latest discovery with the Insight Telescope! Wuhan University's Astronomy Department-led team makes new contributions to black hole research!

On September 1, the world's leading academic journal Science published a long article on the latest research results of an international team led by You Bei from the Department of Astronomy of Wuhan University on the black hole accretion magnetic field. The title of the paper is "Observations of black hole X-ray binaries reveal the formation process of magnetically trapped accretion disks."

This study used observation data from my country's first space X-ray astronomical satellite, Insight, and combined observations from ground-based radio and optical telescopes to find direct observational evidence of the formation process of a magnetically trapped accretion disk around a black hole.

Wuhan University is the first signing unit of this paper, and Associate Professor You Bei is the first author. You Bei, Professor Cao Xinwu of Zhejiang University, and Researcher Yan Zhen of Shanghai Astronomical Observatory are the co-corresponding authors. Researcher Jean-Marie Hameury of Strasbourg Observatory, France, Professor Bozena Czerny of the Polish Center for Theoretical Physics, Wuhan University 2017 undergraduate students Wu Yue and Xia Tianyu, Professor Marek Sikora and Professor Piotr T. Zycki of the Copernicus Astronomical Center of the Polish Academy of Sciences, and Researcher Zhang Shuangnan and Researcher Du Pu of the Institute of High Energy Physics of the Chinese Academy of Sciences are the collaborators.

The physical process of a black hole capturing gas is called "accretion", and the gas falling toward the black hole is called the accretion flow, which is in a plasma state. The viscosity process in the accretion flow can effectively release its gravitational potential energy, partially converting it into radiation energy, generating multi-band radiation that can be observed by ground-based and space telescopes. Therefore, through the accretion of gas, black holes indirectly demonstrate their existence. Observation of these radiations has become an important way to study black holes.

In 2019, the Event Horizon Telescope (EHT) collaboration released the first black hole photo in human history (M87), unveiling the mystery of the black hole and its surroundings that we can "see". However, there are also "invisible" magnetic fields around black holes. When a black hole accretes gas, it also drags the magnetic field inward. Theory holds that as the accreting gas continues to bring in the external weak magnetic field, the magnetic field in the inner region of the accretion flow will gradually increase. Correspondingly, the outward magnetic force of the magnetic field on the accretion flow will gradually increase, and eventually compete with the inward gravity of the black hole. At this point, the accreted matter is trapped by the magnetic field and cannot fall freely and quickly into the black hole's event horizon, forming a magnetic confinement disk. The theoretical model of magnetic confinement disks has been developed very maturely and successfully explains many complex observational phenomena of black hole accretion systems. However, there is no direct observational evidence of the existence of magnetic confinement disks, and how magnetic confinement disks are formed is an unsolved mystery. Many studies have pointed out that there may be magnetic confinement disks around the supermassive black hole at the center of the M87 galaxy. However, even though the EHT's extremely high-resolution observations of M87 obtained information on the magnetic field near its black hole (such as its position), it still failed to confirm the existence of a magnetic confinement disk.

In addition to the supermassive black holes at the center of galaxies, there are also stellar black holes in the universe. At present, astronomers have detected the existence of stellar black holes in many binary star systems, and their mass is generally about ten times that of the sun. The research team used multi-band observation data of the black hole X-ray binary MAXI J1820+070 during the outburst to observe an unprecedented long-time delay phenomenon: the radio radiation of the jet and the optical radiation of the outer region of the accretion flow lagged behind the hard X-rays of the high-temperature gas (hot accretion flow) in the inner region of the accretion flow by about 8 days and 17 days, respectively. The research team pointed out that the weak magnetic field in the outer region of the accretion disk is enhanced by the hot accretion flow around the black hole, and the larger the radial scale of the accretion flow, the more obvious the magnetic field enhancement. By analyzing the X-ray observation data, the research team found that the hard X-ray radiation decreases with the decrease of the accretion rate, while the radial scale of the hot accretion flow expands rapidly with the decrease of the accretion rate, which rapidly enhances the magnetic field near the black hole, thus forming a magnetic confinement disk about 8 days after the peak of the hard X-ray radiation.

This work reveals for the first time the magnetic field transport process in the accretion flow and the complete process of the formation of magnetic confinement disks in the hot accretion flow near the black hole. Therefore, it has become the most direct observational evidence of the existence of magnetic confinement disks to date. Due to the universality of physical processes, this research result will greatly advance the understanding of key scientific issues such as the formation of large-scale magnetic fields in black hole accretion disks of different magnitudes and the mechanism of jet acceleration.

In addition, through numerical simulation of the black hole X-ray binary outburst process, the research team revealed for the first time that when the black hole accretion is about to end, due to the irradiation of hard X-rays, more accreting matter in the outer region will accelerate to fall towards the black hole due to instability, causing optical flashes in the outer region of the accretion flow, with the peak lagging behind the hard X-ray radiation peak of the thermal accretion flow by about 17 days.

China's Smart Eye has made remarkable achievements

Insight is my country's first space X-ray telescope and the final work of the first phase of the Space Science Pilot Project. Its name not only implies that China has a "unique vision" in space, but also commemorates Mr. He Zehui, a female scientist in my country. In June 2017, the Insight Hard X-ray Modulation Telescope was launched into space and reached a near-Earth circular orbit at an altitude of 550 kilometers and an inclination of 43°. After more than half a year of verification and testing, in January 2018, Insight was officially put into use, becoming a powerful tool for Chinese scientists to look at the stars and explore the universe, and also the world's X-ray modulation telescope with the widest spectrum measurement range and the strongest resolution.

As of June 30, 2019, Insight has scanned the galactic plane more than 1,000 times, monitored and announced the long-term flux changes of more than 600 X-ray sources; conducted fixed-point observations of more than 60 X-ray objects of various types, and made important progress in hard X-ray observation research on neutron star magnetic field measurements, quasi-periodic oscillations of black hole accretion, and neutron star thermonuclear bursts; detected more than 170 gamma-ray bursts; and the accuracy of pulsar navigation orbit determination reached the best international level.

Since 2020, many discoveries made by Insight have amazed the world. In February 2021, the Insight satellite confirmed that the X-ray burst associated with the fast radio burst numbered FRB 200428 came from a magnetar numbered SGR J1935+2154 in the Milky Way, and was the first in the world to confirm that the two X-ray pulses contained in the X-ray burst were the high-energy counterparts of the fast radio burst. In August 2022, the Insight (HXMT/Insight) scientific team directly detected the strongest magnetic field in the universe, which is about 60% higher than the previous record of the strongest magnetic field measurement held by NASA's Rossi X-ray Timing Explorer (RXTE).

Comprehensive sources: Wuhan University, CCTV News, Digital Beijing Science Center, Science and Technology Daily, National Space Science Center, etc.