Can black holes devour matter without restraint? Wrong! Learn more about magnetically trapped accretion disks

The vast and deep universe has attracted countless scientists to explore it throughout their lives. The most mysterious of all is the deep "cave" hidden in the darkness - the black hole. As the ultimate celestial body in modern physics, the powerful gravity of the black hole makes it devour everything like a gluttonous beast in the universe, and even light cannot escape. As early as 1783, British scientist John Michell proposed that "if a celestial body has the same density as the sun and is 500 times larger than the sun, then light cannot escape from this celestial body." At the same time, in 1796, French scientist Pierre-Simon Laplace also made a similar prediction in his "The System of the Universe." In 1799, he gave a detailed calculation process and obtained that "a celestial body with the same density as the earth (4 times the density of the sun) and 250 times the size of the sun can imprison light." Einstein's general theory of relativity also predicted the existence of such a celestial body. At a conference in 1967, American physicist John Wheeler called this celestial body a "black hole."

Today, scientists are making progress in exploring the mysteries of black holes. Recently, Chinese astronomers used multi-band observation data to reveal the magnetic field transport around black holes and the formation process of magnetic confinement disks. The research was published on September 1 in the world's authoritative academic journal Science.

01 Can we see black holes? Yes! Accretion allows us to "see" black holes

The matter around a black hole will be "captured" by the black hole due to its huge gravitational force, a phenomenon called "black hole accretion". The accreted matter does not fall into the black hole like a free fall, but forms an accretion disk and rotates around the black hole, and is eventually swallowed by the black hole (imagine the water in your siphon toilet).

Massive stars will eventually evolve into black holes, with masses ranging from several to 20 to 30 times that of the sun. In a binary star system where two stars orbit each other, if one of the stars evolves into a black hole, when the other star is close enough to the black hole, its matter will be accreted by the black hole, and the accretion disk formed will be very hot, with the innermost area even reaching tens of millions of degrees, and will radiate very strong X-rays, so this type of binary star is called a black hole X-ray binary . Of course, their radiation is not only in X-rays, but covers almost all electromagnetic bands.

Astronomers estimate that there may be tens of millions of black holes in binary star systems in our galaxy, but so far, we have only discovered a few dozen . It is possible that most of them do not accrete matter from their companion stars or accrete very little matter, so we "cannot see" them .

Most black hole X-ray binaries are in a quiescent state for a long time, with only a small amount of matter being accreted by the black hole. As matter accumulates in the accretion disk, when it reaches a certain critical point, the instability of the accretion disk is triggered, and it enters an explosive state. A large amount of matter enters the inner area of ​​the accretion disk, radiates strong X-rays, and eventually falls into the black hole. The brightness during an explosive state will increase by tens of thousands or even millions of times compared to the quiescent state.

In March 2018, the black hole X-ray binary MAXI J1820+070, about 10,000 light-years away from us, entered an outburst state. Near the peak of the outburst, it was the brightest X-ray source in the sky (see Video 1). Many telescopes around the world monitored this outburst, including radio, infrared, optical, ultraviolet, X-ray and gamma rays. my country's first X-ray astronomical satellite, Insight, also continuously monitored this outburst. Based on these large amounts of observational data, astronomers are able to study the properties of black holes and their accretion and jets.

Video 1: Each dot is an X-ray source, and the size of the dot represents the brightness. The red dot is the black hole X-ray binary MAXI J1820+070, and the curve below is its brightness change over time (the Milky Way background comes from ESA/Gaia/DPAC).

02 How does the accretion disk move? How much matter can it accrete? The magnetic field around the black hole has a lot of say!

Magnetic fields are everywhere in the universe. Our sun has a magnetic field. The magnetic field in calm areas is about 1-2 gauss, and the magnetic field in sunspot areas can reach 3,000 to 4,000 gauss. A celestial body called a magnetar has the strongest magnetic field in the universe, which can reach 10 to the 15th power gauss.

According to the black hole no-hair theorem, a black hole itself does not carry a magnetic field. The matter accreted by a black hole is usually in a plasma state, which carries a magnetic field. These disordered microscopic magnetic fields can cause matter to lose angular momentum and gradually fall into the black hole.

People believe that there should be large-scale magnetic fields around black holes. One of the important reasons is that the generation of jets requires large-scale magnetic fields. Charged particles move in such large-scale magnetic fields, and the radiation generated will be polarized. Through radio polarization observations, we know that there are indeed large-scale magnetic fields in the jets of supermassive black holes.

In the polarized photo of the supermassive black hole at the center of the M87 galaxy released in 2021, it was found that there is also a large-scale magnetic field near the black hole (Figure 1). For black hole X-ray binaries in the Milky Way, although they are closer, our current telescopes cannot image them because the black holes are too small.

The presence of a large-scale magnetic field will have a significant impact on the accretion flow near a black hole, such as the radial velocity (radial migration is movement toward the black hole) and the accretion rate (the mass accreted per unit time) , and will also produce a strong outflow (matter moving outward). Therefore, the presence of a magnetic field will bring about many other observational effects.

Figure 1. Polarized observation images of the supermassive black hole and its jet at the center of the M87 galaxy. The curved bright stripes show the distribution of large-scale magnetic fields. The top is a jet on a scale of 1,300 light years, the middle is a jet on a scale of 0.25 light years, and the bottom is an image of a scale of 0.0063 light years (400 AU) around the black hole. (Image source: EHT Collaboration; ALMA (ESO/NAOJ/NRAO), Goddi et al.; VLBA (NRAO), Kravchenko et al.; JC Algaba, I. Martí-Vidal)

03 "Crazy" accretion: the formation of magnetically trapped accretion disks

Astronomers generally believe that there is a relatively weak large-scale magnetic field in the outer region of the accretion disk, which will move inward with the accretion flow. Since black holes only "swallow" matter and not "swallow" magnetic fields, the magnetic field will accumulate near the black hole and gradually increase.

There is a type of accretion flow called "advecction dominated accretion flow, ADAF". As the name suggests, in this accretion flow, matter falls into the black hole very quickly. The weak external magnetic field will be dragged by the accretion flow and rapidly increase near the black hole. When the electromagnetic force of the magnetic field on the accreting matter can counteract the gravity of the black hole, the speed at which matter falls into the black hole will be greatly reduced, perhaps to only one tenth or even one hundredth of the original speed, just like being trapped by the magnetic field. This is called a magnetically arrested disk (MAD) - a "crazy" accretion flow .

Researchers from Wuhan University, Zhejiang University, and the Shanghai Astronomical Observatory of the Chinese Academy of Sciences analyzed multi-band data from the outburst of the black hole X-ray binary MAXI J1820+070, revealing the process of gradual expansion and magnetic field transport of the radially-dominated accretion flow. Calculation results show that its size expanded 30 times in just over ten days. The larger the size, the easier it is to quickly bring the external weak magnetic field into the inner area of ​​the accretion flow, ultimately making the magnetic field strength in the innermost area reach the standard of a magnetically trapped accretion disk (Video 2).

Video 2: As the radially-dominated accretion flow expands, the magnetic field is rapidly brought into the inner region of the accretion flow, eventually forming a magnetically trapped accretion disk (MAD). The video is from the paper https://doi.org/10.1126/science.abo4504.

Black holes, seemingly irrelevant to daily life, are one of the key issues in studying the origin, evolution and structure of the universe, and have profoundly affected the progress of science, technology and even human production and lifestyle. Today, human understanding of time and space is far from over. The exploration of the unknown is driving scientists to bravely try to break the shackles of thinking about the limits of the universe, thus producing more great discoveries.

Author: Yan Zhen, researcher at Shanghai Astronomical Observatory, Chinese Academy of Sciences

Reviewer: You Bei, Associate Professor, School of Physical Science and Technology, Wuhan University

Produced by: Science Popularization China

Produced by: China Science and Technology Press Co., Ltd., China Science and Technology Publishing House (Beijing) Digital Media Co., Ltd.