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When it comes to black holes, people usually think of the shape with an accretion disk. An accretion disk is like a black hole dinner plate, and its presence means that the black hole meal is not over yet. Because accretion disks emit a lot of radiation, almost all black holes observed so far are eating black holes.

However, there are always some black holes, which may be short of food or have enough to eat. In short, they have stopped eating and are in a state of temporary dormancy. Without an accretion disk, these black holes find it difficult to expose themselves, and they lurk in the universe like ghosts, waiting for the next prey to come.

Black holes, although they sound powerful, usually grow bigger from small to small. There are two main ways to get bigger: one is to merge directly with other black holes, and the other is to feed yourself up little by little through an accretion disk.

Accretion disks are not unique to black holes. Many celestial bodies such as neutron stars and white dwarfs can have their own accretion disks. That is to say, these dense celestial bodies often use their strong tidal forces to tear up nearby prey, and then put the pieces around them to enjoy them slowly. This phenomenon is called "tidal destruction event (TDE)".

Of course, there are some different, for example, when the prey's trajectory is relatively flat, the black hole will wait for it to take a bite every time it approaches, and then bite again.

For example, Swift J0230, which lies 500 million light-years away, has a black hole 200000 times the mass of the sun, nibbling on the companion star next to it on average once a month, each equivalent to about three Earths.

This process is somewhat similar to the predation process of predators. But who is the hunter and who is the prey? it doesn't look at size or strength here. It is not that whoever has a great mass and gravity is strong. For celestial bodies, the key factors that become hunters are density and self-strength. This is why a white dwarf or neutron star with a smaller mass can cannibalize a companion star with a much larger mass.

From the word "nibbling", it can be seen that this way of eating in an accretion plate is usually slow. If only gravity was considered, the matter (mainly gas) in the accretion disk would not even fall into the black hole. Because the rotation of these matter around a black hole is like the rotation of a planet around a star, the inertial centrifugal force generated by the rotation will counterbalance the gravity centripetal force.

In addition to the centrifugal force, the magnetic field generated by the accretion disk on the side near the visual interface will also help this confrontation. Because as it rotates closer to the center, the material in the accretion disk continues to be compressed, and the resulting magnetic field becomes stronger and stronger. The centrifugal force is almost too much to bear, but the electromagnetic force helps again, and the matter here is imprisoned in this area like a quantum lock in a maglev, which is called a magnetic confinement disk (MAD).

By the way: although the theory of magnetic confinement disk is mature, it has been lack of practical observation evidence. Not long ago, however, Chinese scientific research team, together with foreign teams, used data from China's "discerning" X-ray astronomical satellite and ground telescopes to find direct evidence of the formation of magnetic traps around black holes for the first time. The results have been published in the journal Science.

So why does the matter in the accretion disk end up in a black hole?

In fact, it is the same as planetary orbits: the closer the planet is to the star, the shorter the period and the faster it rotates; the farther away from the star, the longer it orbits and rotates more slowly, and so is the gas in the accretion disk. The difference is that the planets are very sparse and have little intersection with each other except for point gravitational pull; but the accretion disk is different, where the gas is very dense, and the difference in rotational speed between the inner and outer rings will cause severe friction and collision between gas particles, which is called "viscous action".

The gas particles dissipate their energy through friction, so the speed slows down. The rotational speed slows down, the centrifugal force decreases, and gravity prevails again, so the particles are slowly sucked away by the black hole. Most of the energy dissipated by friction becomes heat, which is eventually released in the form of thermal radiation.

So the whole accretion process, macroscopically, is the process in which the gravitational potential energy of gaseous matter is converted into thermal energy and radiated outward. This is the main reason why black holes can be discovered by us at present.

However, the black hole hunting process is not always so slow, in the face of some dense "little guy", its hunting process can be as fleeting as a tiger prey.

For example, scientists have used computers to simulate the situation when a neutron star is torn apart by a black hole, and the whole process can be said to be as fast as lightning. In just a few milliseconds, the neutron star is instantly torn apart by the black hole, 80% of the matter is immediately swallowed up, and the remaining small amount of matter is quickly sucked up and squeezed out after becoming the accretion disk of the black hole, and the whole process takes only 1-2 seconds.

Not only dry rice is fierce, some black holes also eat a lot. For example, the Weber Telescope captured an "ancient giant" 11.5 billion years ago-a quasar (SDSS J165202.64172852.3).

We know that a quasar is an active galactic nucleus (AGN) at the beginning of galaxy formation, and its center is a supermassive black hole that is frantically eating. The prey of a constant star black hole is usually companion stars such as stars, white dwarfs and neutron stars, but the prey of this quasar is actually three young galaxies! It can be predicted that in the future, all three galaxies are likely to die in the mouth of the gluttonous beast, and it is only a matter of time left.

Stuffing things into your mouth is bound to cause indigestion, and black holes have symptoms such as vomiting and hiccups from time to time.

Due to the distortion of the magnetic induction line of the rotating accretion disk, when accumulated to a certain extent, some matter that has not yet been swallowed by the black hole will be ejected along the rotational axis of the black hole to the poles, up to tens of thousands or even hundreds of thousands of light-years in length. The speed of this jet can reach a fraction of the speed of light, so that there will be a significant relativistic effect, so it is also known as the "relativistic jet".

In addition to vomiting while eating, black holes burp from time to time. For example, astronomers observed a tidal destruction event (AT2018hyz) in 2018, but its jet signal was not observed until two years later. Why did you burp today after eating two years ago? Scientists speculate that it may be a change in the internal state of the accretion disk, or it may be related to the interaction of the surrounding environment, but the reason is still unclear.

Thus it can be seen that as long as the accretion disk is still there, the feeding process of the black hole is not completely over. Only when the accretion disk is almost invisible does it mean that you have enough to eat and can have a rest. This kind of black hole is called "dormant black hole".

Without the accretion disk emitting radiation, the dormant black hole is like a beast hiding in the grass, which is difficult to detect. The last time astronomers discovered a dormant black hole was in the large Magellanic Galaxy 5000 light-years away. At that time, through the analysis of a star, it was found that its strange orbit indicated that there should be an invisible massive companion star. After ruling out all possibilities, it was concluded that the invisible companion star should be a constant star black hole with nine times the mass of the sun.

However, in 2022, through the Gaia Telescope, astronomers discovered a dormant black hole in the Milky way, Gaia BH1, only 1560 light-years from Earth, which is currently confirmed to be the nearest black hole.

Gaia BH1 is a constant black hole with nearly 10 times the mass of the sun, which lies in a binary system with another sun-like companion. Unlike other binary systems, this binary system has a very wide orbit and the companion star is very far away from the black hole. But even so, in terms of the volume of this black hole, the maximum radius of its predecessor star at the end of its evolution is even much larger than the current orbit of its companion star. In other words, this companion star may have been surrounded by another super-large star. If so, how did it survive? With its petite body of solar mass, it is impossible to escape from a red supergiant with tens of times the mass of the sun.

It was thought to be an isolated case, but a second similar black hole, Gaia BH2, was discovered 3800 light-years away. BH2 is also in a binary system in which the black hole is also similar in size, nearly nine times the mass of the sun, except that the companion star is a red giant with a mass similar to that of the sun and approaching twilight years.

The findings of BH1 and BH2 not only show that the current binary evolution model needs to be further improved, but also make us realize that such dormant black holes lurking in the universe may only be difficult to detect, in fact, they are not rare in the vast interstellar space.

Wait a minute

First found outside the Milky way and now found in the Milky way, will one day we suddenly find a dormant black hole (or a very small primary black hole) in our solar system?

In recent years, more and more astronomers have realized that objects outside Neptune in the Kuiper belt, such as Sedna, have surprisingly flat orbits, which means that in addition to Neptune's gravity, they may also be affected by some unknown gravity. According to theoretical calculations, the unknown source of gravity may be a celestial body at least twice the mass of Earth, which is called "Ninth Planet" or "Planet X."

Since no one has ever seen this unknown object, some scientists speculate that it may be a very small primordial black hole. If this is the case, there may be another possibility for the future fate of the solar system.

references

[1] https://www.science.org/doi/10.1126/science.abo4504

[2] https://www.nature.com/articles/s41550-023-02073-y

[3] https://www.esa.int/Science_Exploration/Space_Science/Webb/Webb_uncovers_dense_cosmic_knot_in_the_early_Universe

[4] https://en.wikipedia.org/wiki/Astrophysical_jet#Relativistic_jet

[5] https://mp.weixin.qq.com/s/bguqJfW_3vZFTK5fU46AYA

[6] https://www.esa.int/Science_Exploration/Space_Science/Gaia/Gaia_discovers_a_new_family_of_black_holes

[7] https://en.wikipedia.org/wiki/Gaia_BH1

[8] https://academic.oup.com/mnras/article-abstract/518/1/1057/6794289

[9] https://academic.oup.com/mnras/article/521/3/4323/7093135

[10] https://noirlab.edu/public/news/noirlab2227/

This article comes from the official account of Wechat: Linvo says ID:linvo001, author: Linvo

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