Today I will talk to you about tidal disintegration. Speaking of tidal disintegration, even if you are not familiar with this field, you should be able to guess the name alone. When it comes to tides, the eight achievements are related to the action of tides, and disintegration is shattering. It's just not clear who the sloppy unlucky egg is. Let me introduce you to you.
Let's start with what we're familiar with. When it comes to tides, everyone's first reaction may be the tides and ebbs and flows that we are familiar with. As an ordinary little broken ball, our Planet is also subject to gravitational pull from other celestial bodies. We know that gravity decays with the square of distance, and the farther away the distance, the weaker the gravitational force. So take the moon as an example, because the earth is a sphere, there is a difference in the size of the gravitational action of the moon in some areas of the earth that are closer to the moon than in the farther regions. In this way, the gravitational force of different regions is also different. Originally, if the whole earth was a solid scale, this difference in gravity might not be able to turn over any big waves. However, most of the earth is covered by very mobile seawater. As the earth rotates, the sea will rise and fall with different forces, that is, we see the tide rising and falling. In the solar system, the objects that have a clear tidal effect on the Earth are the sun and the moon. The sun is massive but farther away from us, and the moon is small but close. Put two small partners together to contribute to Bibi, and you will find that the moon has a greater impact.
Figure 1. High-resolution images of Io obtained by the Galileo probe in July 1999. A volcanic plume can be clearly seen erupting from The surface of Io.
（图源：NASA/JPL/University of Arizona）
You might think that tidal action is not extremely vicious except for occasionally photographing some boats and marine life on the beach. Moreover, it has also brought about a poetic change in our world, making our lives less monotonous and seemingly quite good. However, I am afraid that this is only because we are destined: the moon is relatively small in mass and far enough away from us. What if it weren't? Just look at poor Ganymede. This unlucky egg is neighboring Jupiter, the largest planet in the solar system, and it is a particularly close one. As a result, due to severe tidal action, this moon, which is only slightly larger than the moon, was repeatedly rubbed by Jupiter on the ground, and was indisputably kneaded into an ellipsoid. And because the tidal action was so strong, Io was severely pulled inside, generating a lot of heat, resulting in more than four hundred active volcanoes on it. Volcanic activity is so frequent that it is almost impossible to find a lunar crater on it, because the newly impacted crater will soon be wiped away by the material from the volcanic eruption, and it will be a hell to get rid of it (see Figure 1).
Therefore, tidal action can also have a strong destructive force. So what if the situation is a little more extreme? In order to align our discussion with mainstream astronomical science articles, it is necessary to invite out an old friend familiar to the broad masses of the people - black hole. You've more or less heard of some of its legends, which are very extreme objects, and any matter that falls into a black hole can't turn back. If our sun had become a black hole (which is actually impossible from the point of view of stellar evolution), and the Earth had run so close to the black hole, it would have been entirely possible that the tidal action near the black hole would have torn the Earth to pieces. However, this deadly distance needs to be very, very close, so close that it is actually only a little more than half of the sun's radius. Such a close distance is certainly unlikely. But the universe is so big, what bird doesn't have. Similar things, it is not necessarily impossible to change the environment.
We have found evidence of the existence of supermassive black holes at the centers of many galaxies. For example, at the center of our Milky Way galaxy there is a black hole with about four million times the mass of the Sun. And there are many stars in the center of the Milky Way. If a star had run to provoke the central black hole, the tragedy assumed earlier would have been possible. For example, if our sun runs to the center of the milky way to visit the door, and accidentally gets too close to the supermassive black hole, then the tidal action of the black hole may tear the sun to pieces. Only this time, the lethal distance was much greater—about a tenth of the distance between the sun and the earth. This is the tidal disintegration process that we are going to talk about today.
Why would astronomers be interested in such a thing? I'm afraid this has to start in the 1950s and 1960s. At that time, people gradually discovered many strange celestial bodies. Although these objects look like a small bright spot like a star, they can reach hundreds of times brighter than ordinary galaxies. That is, these celestial bodies are very far away from us. Apparently, out of some unknown mechanism, they produce very large amounts of radiation. We call these celestial bodies quasars. The question that arises is, where does the energy released by these celestial bodies come from? Various models have been proposed, one of which is widely accepted as the central engine of the quasar is a supermassive black hole that devours surrounding matter. This theory explains the source of energy mechanically, but it was not clear at the time where the engulfed matter came from. You know, due to the existence of angular momentum, the fact that matter falls into a black hole is not as easy as in science fiction movies. In fact, it's much harder to fall inside than to hang out. To solve this problem, the American astronomer Hills proposed a seemingly reliable mechanism in 1975, that is, the tidal disintegration we mentioned earlier. As long as there are stars constantly being tidally disintegrated by the central black hole, the energy source of the quasar is not a problem. So in the late seventies and early eighties, astronomers did a lot of work to estimate the probability of such events, to see if tidal disintegration could provide enough energy for the quasars. However, after a lot of work, they realized that the chances of such an event occurring were too low. For a galaxy, it may take tens to millions of years for a tidal disintegration event. Why is it so low? Simply put, getting a star to fall near a black hole and disintegrate is as difficult as throwing a glass ball and dropping it in a one-meter-square bunker thousands of kilometers away. If there weren't a lot of stars in the center of the galaxy, such an event would probably not be waiting until the sky and the earth were old. Obviously, for a supermassive black hole at the center of a galaxy, tidal disintegration is at best an after-dinner dessert, and the staple food has to be found elsewhere. Realizing this, the study of tidal disintegration was relatively silent for several years.
But in 1988, the British astronomer Rees published an interesting article. His idea is simple, although tidal disintegration is not enough to provide the energy required by quasars, such events themselves also produce strong bursts of energy that can be observed by us. While the theory estimates that the probability of such events occurring within a single galaxy is very low, given that such eruptions are relatively bright, they should be visible even at greater distances. As a result, due to the large number of galaxies in the detectable range, our overall detection rate of tidal disintegration events should not be very low.
Rees is a well-known astronomer with a keen intuition. All the physical processes involved in his model are drastically simplified. He hypothesized that the fragments of the star after being torn apart by the black hole would move along kepler orbits of different energies, and that the interactions between the fragments would be completely negligible. In this way, about half of the debris will rush directly into the vast space, while the other half will fall back near the black hole. He also hypothesized that the falling debris would quickly form a disk-like gas structure near the black hole— an accretion disk— and then slowly be swallowed up by the black hole. This will then see a flare event that is mainly concentrated in the soft X-ray to ultraviolet band, whose luminosity decays with time to the power of -5/3, and which can be observed on a month-to-month time scale based on the distance from us to us (Figure 2).
Figure 2. The image above is an artistic visualization of the tidal disintegration process, in which part of the disintegrated star remnants run away, and the rest are caught back by the black hole. The figure below shows an observed tidal disintegration event. On the left is the post-eruption observed in the X-ray band (nothing can be seen before the eruption), and on the right is the eruption that should be seen in the optical band (the white circle corresponds to the size of the X-ray source in the left panel).
（图源：Illustration: NASA/CXC/M.Weiss; X-ray: NASA/CXC/MPE/S.Komossa et al.; Optical: ESO/MPE/S.Komossa）
The whole model is very simple, and an undergraduate student who has studied university physics can deduce it. Of course, we also know that the interactions between gases are actually very complex, and can rees make so many assumptions to get a "beggar's version" model that can realistically reconstruct the whole event? More than a year later, someone did a calculus using a numerical simulation of fluid dynamics containing gas interactions, and found that it was basically consistent with his predictions. Thus, there was a small climax in the study of this question. Soon, however, patience was lost because the observations expected by the theory never found.
Figure 3: X-ray band optical curve of the first tidal disintegration event. The various markers in the figure are X-ray flow rates measured by different detection equipment at different times. The red dotted line is a theoretical light curve based on Rees's classical model, which is visible and very well observed. The abscissa units in the figure are the time in years.
(Source: Komossa 2005)
And just like that, another decade has passed. By the turn of the century, the German astronomer Komossa and his collaborators had finally found such a flare event in the roentgen satellite observations, and its light curves were well matched by theoretical predictions (see Figures 2 and 3). At this point, after more than 20 years of unremitting efforts, astronomers have finally pulled out the tidal disintegration. Astronomers are excited about this discovery. Because tidal disintegration is a very useful probe. In general, extreme objects such as black holes do not have significant radiation on their own. If we want to study black holes, we need to find a mechanism that can illuminate black holes. This is the case with the aforementioned quasars — large amounts of gas forming an accretion disk around the black hole, while producing strong radiation. Similar situations are found in X-ray binary stars. The so-called X-ray binary stars are generally stellar-mass black holes and companion star formation systems, black holes swallow the material of the companion star to form an accretion disk and bring strong X-ray radiation. visible. To "see" a black hole, an accretion disk is essential. The problem is that most of these kinds of things that happen in the universe have very long lifespans, like the quasars we mentioned, that can even last 10 million to 100 million years. So it is difficult for us to see the formation and dissipation of accretion disks. Therefore, there are many unsolved problems in the evolution of accretion disks. The tidal disintegration process is precisely a relatively short-lasting event, which can not only temporarily illuminate those black holes that are usually very quiet due to the lack of gas, but also give us the opportunity to trace the formation and evolution of accretion disks from the beginning until the end of their fate. This is very helpful for testing the accretion disk theory.
Figure 4. Above: Supermassive binary black hole tidal disintegration art image of galaxies orbiting each other. The main black hole tears a star into an elongated stream of gas that is heated at high temperatures to produce X-ray radiation as it flows toward the black hole to form an accretion disk that rotates around the black hole. When the secondary black hole orbits near the gas stream (without passing through), the resulting destructive gravitational disturbance causes some of the gas in the gas stream to fly away, leaving a gap. The X-ray light curve correspondingly falls abruptly until it goes dark (plot: ESA-C. Carreau)
Below: Complete reconstruction of the X-ray light curve of the binary black hole pair galaxy SDSS J120136.02+300305.5 (solid red line). The diamond symbol in the figure is the observed value, and the downward arrow represents the upper flow limit obtained when the brightness of the X-ray source is below the detection limit, indicating that the actual brightness is lower than this value. The black dotted line is a typical light curve for the tidal disintegration of a single black hole.
(Source: Liu et.al. 2014）
Of course, if there is only this use, it is a small look at it. For supermassive black holes, the location where the star is torn apart is often very close to the black hole's event horizon, so such events are often accompanied by strong gravitational wave radiation. Some of these extreme events are likely to be detected by future space gravitational wave observatories. The study of such events will open a window for us to test the applicability of general relativity under extreme conditions. In addition, we know that galaxies tend to undergo multiple mergers over the course of their long evolution. If most galaxies have a supermassive black hole at their center, the merger of galaxies is likely to create a binary system of supermassive black holes. And if one of the black holes undergoes a tidal disintegration, then due to the perturbation of the other black hole, the remains of the disintegrated star may not be able to fall back to the original black hole continuously, so we will see obvious truncation in the light curve of the tidal disintegration event. Because the strength of this perturbation is also affected by the relative position of the orbit of the double black hole, the truncated material after a period of time may fall back. A team led by Professor Liu Fukun of Peking University was the first to propose this theory and successfully found an observational candidate in 2014. Then, in 2020, Professor Shu Xinwen of Anhui Normal University and his collaborators found a second candidate. As a result, tidal disintegration has become one of the few means of finding supermassive double black holes in tranquil galaxies.
Figure 5. Numerical simulations of relativistic hydrodynamics study the formation of accretion disks after stars are tidally disintegrated by tides. The four sub-plots a-d in the figure represent different stages of evolution.
(Source: Kimitake et. al. 2016）
It is precisely because tidal disintegration is so important that after entering the twenty-first century, especially in the past decade or so, astronomers have made a lot of theoretical research and observations. The results are also very rich. As observational methods advanced, more and more attention began to be paid to events that changed within relatively short time scales, the so-called time-domain astronomy. Although still in its infancy, efforts in this area have already yielded a great deal of observational results. Today, we have found more than 100 candidates, and found that in addition to X-rays and ultraviolet bands, many tidal disintegrations also have very obvious light changes in the visible light band. Today, we find even a lot more tidal disintegration candidates in the visible-light band than in the X-ray/UV band. Moreover, in a few events, we have also observed jets, radio radiation, emission lines and infrared dust echoes that often appear in the nucleus of active galaxies (first discovered by Jiang Ning and his collaborators at the University of Science and Technology of China), and may also detect neutrino radiation.
Finding so many candidates, of course, has also brought a lot of problems. For example, it has been found that in fact most candidates, especially those found optically, have very different optical curves than the theoretically expected rate of decay. Some candidates' optical light changes are not even synchronized with X-ray light changes. This shows that the simplified model we are familiar with is not sufficient to explain all observed phenomena. There is still a lot of confusion about the formation and radiation mechanism of the tidal disintegration accretion disk. As a result, a large number of detailed theoretical and numerical simulation work has discussed in depth the effects of the mechanisms existing in various real-world situations on observation, and also proposed a geometric configuration theory similar to the unified model of active galactic nuclei. The study of tidal disintegration quickly entered an explosion in which observation and theory went hand in hand.
Tidal Disintegration, as a relatively young field, has continued to evolve over the past forty years, slowly evolving from an inconspicuous hypothesis into a lively family. Some say that tidal disintegration is in a golden period of development. And I'm afraid it prefers to paraphrase Zhou Runfa: "Success? I'm just on the road. "In the near future, a large number of time-domain sky survey projects will expand the sample of tidal disintegration by several orders of magnitude. For example, the Vera Rubin Observatory, which will be operational next year, can scan observable space zones every few days and is expected to find hundreds of tidal disintegration events each year. The Einstein Probe X-ray Sky Survey Telescope developed by China will also be launched around 2024, and it can also be scanned every few days in the X-band. So in the near future, astronomers are about to usher in a feast of tidal disintegration research.
Figure 6. The future of tidal disintegration detectors: Rubin Observatory (left) and Einstein probe (right)
Introduction to the Presenter
Li Shuo is an assistant researcher at the National Astronomical Observatory of the Chinese Academy of Sciences. He received his Ph.D. in Astronomy from the Department of Astronomy, School of Physics, Peking University in 2012. His research interests are in the evolution of gravitational systems, and his main interest is the co-dynamic evolution of supermassive black holes and galaxies.
Rotating Editor-in-Chief: Ran Li
Editor-in-charge: Yuan Fengfang
Editors: Zhao Yuhao, Qiqi
National Astronomical Journal of China, November 2021
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