There has never been a black hole so close to the earth

 Black holes are undoubtedly the most mysterious celestial bodies in the universe. They hide in the darkness and swallow all incoming light. Regardless of size, all black holes have a common feature, which is the event horizon. Once the light crosses the event horizon, it cannot escape. However, the black hole will still show its feet, and we will catch the trace: when there is a star not far from the black hole, the star’s matter is attracted by the black hole, and the ultra-high temperature dust and gas form an accretion disk that rotates rapidly around the black hole, releasing it. The X-rays can "light up" the black hole. So, what if there is no such companion star close enough around the black hole? Although it cannot be seen directly, scientists can indirectly detect these mysterious celestial bodies through the gravitational effect of black holes on other celestial bodies.

In a new study published in the Journal of Astronomy and Astrophysics, astronomers at the European Southern Observatory (ESO) made a remarkable discovery through this method-they have found the distance from the Earth so far The nearest black hole. If the conclusion is true, people in the southern hemisphere can even see the star system where the black hole is located without the aid of observation equipment.

　　 The star system HR 6819 is located in the telescope constellation in the southern sky, 1000 light years away. This is an ordinary binary star system that is unremarkable and can barely be seen with the naked eye in the southern hemisphere, but the researchers found that the spectral lines of one of the stars show periodic swings. Therefore, in this system, there must be an undiscovered celestial body.

　　 Further analysis pointed out that one of the stars seems to revolve around this invisible celestial body with a cycle of 40 days; the other star is farther away and has a longer cycle. The mass of this celestial body is equivalent to 4.2 times the mass of the sun. If this is a star, then considering its mass and distance, observers on Earth must be able to see the star. Therefore, the study suggests that the center of the HR 6819 system is a black hole.

　　 15 years of data buried

　　 The lead author of this study, Thomas Rivinius of ESO, said: "We initially thought that HR 6819 is a binary star system, but after further examination, we discovered that it is actually a system composed of three celestial bodies."

　　 The research team used ESO’s 2.2-meter telescope at Lasila Observatory in Chile to make this discovery. However, it should be noted that this observation was not made recently. As early as 2004, astronomers obtained these data during several months of observations. But these data did not attract the attention of astronomers-as mentioned above, they think this is just an ordinary binary star system.

　　 Until last year, the Liu Jifeng team of the National Astronomical Observatory of China used the Guo Shoujing telescope to discover the largest-mass constant-star black hole with a mass 70 times that of the sun. Liu Jifeng’s team noticed the abnormal spectral lines in a star, and then through further analysis, the star was moving approximately circularly around an "invisible celestial body."

This research published in the journal Nature caught Rivinius’ attention: “I think, wait, I have some unused data in my drawer, and LB-1 (the double star composed of black holes and stars discovered by Liu Jifeng’s team) System) is very similar.” So, the Rivinius team began to re-analyze these dusty data. (Note: Just last week, two articles in the "Nature" magazine launched a dialogue on the constant star black hole in the LB-1 system. Scientists at the University of Leuven in Belgium believe that the Liu Jifeng team’s explanation of the hydrogen spectrum is not Correct, so the source of the hydrogen emission line is not a black hole; and Liu Jifeng’s team responded in another study that the problem of the hydrogen emission line is more complicated than previously expected. Combined with the latest data, the black hole in LB-1 may have a mass Between 23 and 65 times the mass of the sun.)

　　 According to the ages of the two stars of the HR 6819 system, Rivinius et al. proposed that the black holes in the system are the remains of supernova explosions 10 million years ago. Under normal circumstances, the black hole in the binary star system can accumulate the material of the companion star, and the large amount of X-rays released thereby form a luminous disk, which surrounds the black hole. Interestingly, in the HR 6819 system, the distance between the two companion stars and the black hole's orbit is far enough to avoid being "eaten" by the black hole. Because of this, this "double star" system was once forgotten by astronomers for many years after being observed.

　　Astronomical significance and controversy(https://www.branddesign.cc)

　　 If the black hole in the HR 6819 system does exist, this discovery has a series of astronomical significance besides updating the closest distance between the black hole and the earth. It was previously believed that supernovae would affect the orbits of prominent surrounding stars and even "kick" the stars into interstellar space. And this stable three-body system tells a completely different story. "From this case we learned that black holes can be formed without ‘kicking’ surrounding stars." Rivinius said.

　　 Another significance of this research is that it shows that such "quiet" black holes are more common than previously expected, so there are still a large number of similar unknown black holes waiting to be discovered. Even LB-1 may belong to this kind of previously unknown black hole system. Of course, due to the longer distance and weaker signal, the observation of LB-1 is more difficult, but it is not impossible. Rivinius said that they have planned to study the LB-1 system.

　　HR 6819 also provides tantalizing clues for understanding the formation mechanism of the black hole system that produces gravitational waves. When two black holes, or black holes merge with a neutron star, the ripples in space and time have been detected by astronomers. But before the merger, how such a system was born is an unsolved mystery.

　　 The scientific significance of this discovery is full of temptations, but some scientists have also put forward different opinions. Kareem El-Badry of the University of California, Berkeley (who was not involved in the study) believes that the conclusions of this study are "obviously questionable." El-Badry noted that this conclusion is based on a series of assumptions, especially the stars in the system that are closer to the black hole, and the mass is about 5 times that of the sun. In his opinion, this assumption is not sufficiently reliable. Once the mass of this star is less than the Rivinius team's hypothesis, the mass of the invisible object in the center of the system will also decrease. Therefore, this celestial body may not be a black hole at all, but a star with insufficient mass to be observed. El-Badry said: "There must be a certain celestial body there, but it is unwise to say that it is a black hole."

Edward van den Heuvel of the University of Amsterdam questioned from another angle. He believes that it is currently impossible to determine whether the celestial body recognized as a black hole is a single body with 4.2 times the mass of the sun, or two very close, with an average mass of 2.1 times the sun Mass stars. "This may also be a four-body system. Many bright stars in the universe are located in such a four-body system," van den Heuvel said. "If the celestial body emits X-rays at a certain moment, then we can be sure of it. It's a black hole. But if we still can't observe the X-ray signal, then the question is: Is it a black hole or two stars orbiting each other?"

　　 In this regard, Rivinius believes that if this is a four-body system, then from the light released by HR 6819, we can find features that can clearly identify its identity. To clarify its identity, more telescopes are needed to join in and conduct longer observations. Due to the global spread of the new crown epidemic, a large number of observatories are closed. Rivinius said that they will try immediately after the observation restarts. As for the present, we can say that there seems to be an unknown "dark" partner lurking in the "backyard" of the solar system.