Physicists discover first 'triple black hole' - Why is this phenomenon so rare?

Author: Gou Lijun

Black holes have always been a popular topic among the public. In the universe, stellar black holes are formed by the core collapse of massive stars, and are often produced through violent supernova explosions or milder "direct collapse". The former will cause the black hole or neutron star to obtain a relatively obvious "birth kick", that is, the black hole obtains additional speed through the asymmetric explosion at the time of its birth. However, whether black holes generally have birth kicks and their impact on surrounding celestial bodies remains an unsolved mystery. Recently, scientists' research on the V404 Cygni system revealed for the first time the triple system structure of stellar black holes, bringing new challenges to the classical black hole formation theory. Hello everyone, I am Gou Lijun from the National Astronomical Observatory of the Chinese Academy of Sciences. Today we will talk about the recent black hole discoveries.

We just mentioned that the two main models for the origin of stellar black holes are supernova explosions and direct collapse. A supernova explosion is a violent explosion triggered by gravitational collapse of a massive star after it has exhausted its nuclear fuel. Due to the asymmetry, it produces a strong shock wave, which usually causes the black hole or neutron star to receive a considerable "natal kick". However, this explosion will also throw the surrounding celestial bodies out of the system. Relatively speaking, direct collapse is a more gentle collapse process that does not produce violent shock waves or kicks, and thus may keep the nearby companion star relatively stable under the action of gravity.

Most of the known black hole systems in the universe are binary systems, that is, the black hole and the companion star coexist in a gravitationally bound structure. For example, the first discovered black hole binary system, the Cygnus X-1 system, is a typical black hole companion binary system. The widespread existence of binary systems provides clues for understanding the evolution of black holes, but triple systems are very rare. For example, the familiar Proxima Centauri is in a triple system, but triple systems containing black holes are very rare. However, just recently on October 23, a research result was published in the well-known academic journal Nature. Astronomers confirmed for the first time that V404 Cygni is a triple system containing a black hole.

V404 Cygni is located in the constellation Cygnus, one of the 88 constellations in the sky, about 7,800 light-years from Earth. It was previously considered to be a typical low-mass X-ray binary, consisting of a black hole and a close companion star with a mass less than that of the sun. The companion star orbits the black hole closely, with an orbital period of only about 6.5 days. However, the latest observations have found that in addition to the previously believed system, there is also a distant companion star about 3,500 AU away from the black hole. This makes V404 Cygni a hierarchical triple system: the black hole and the close companion star form a core binary, while the distant companion star orbits this core binary star. This structure is very rare, and the black hole of the core binary star did not disperse the distant companion star due to the birth kick when it was formed, indicating that the black hole may have formed in a low-kick or no-kick collapse process. Studies have shown that the kicking speed of the black hole is less than 5 km/s, which is not enough to cause the distant companion star to escape from the system. It is speculated that the black hole may have been formed through a direct collapse process, and its progenitor star did not experience a violent supernova explosion, but rather gently collapsed into a black hole. This low birth kick prevents the black hole from disturbing the orbit of the outer companion star when it is formed, supporting the direct collapse model. It should be noted here that a few years ago, when we studied Cygnus X-1, we also found that the progenitor star of Cygnus X-1 did not experience a violent supernova explosion, but only ejected very little matter, and finally collapsed to form the black hole of Cygnus X-1. From this point of view, the formation process of the two black holes is similar.

The research team also found that the distant companion star in the system has evolved away from the main sequence star stage, suggesting that the system was formed 3 to 5 billion years ago. In the long time after its formation, the black hole accreted about 0.5 solar masses of matter from the core companion star, causing the core binary star to gradually evolve into its current state. This stable hierarchical triple star structure is consistent with the low birth kick theory, further verifying that black holes can be formed by direct collapse.

The discovery of V404 Cygni has posed a profound challenge to the classical theory of black hole formation. According to the traditional supernova explosion model, when a massive star forms a black hole, it is accompanied by a violent shock wave, making it difficult for the surrounding stars or other celestial bodies to remain gravitationally bound. However, the distant companion star of V404 Cygni was not violently disturbed during the black hole formation process, indicating that the black hole did not experience a significant "birth kick". This means that the direct collapse model can be applied to explain the characteristics of the low birth kick of stellar-mass black holes.

In addition, the hierarchical triple system structure of V404 Cygni supports a model of low-mass X-ray binary formation involving three-body evolution. In this model, the distant companion star causes the core binary star's orbit to gradually narrow through gravitational interaction, eventually forming a compact low-mass X-ray binary. This gravitational interaction will gradually narrow the inner orbit, thereby maintaining the stability of the system.

The discovery of V404 Cygni provides valuable new insights into the formation mechanism and evolution path of black holes. The discovery shows that stellar collapse black holes may be formed through direct collapse with a very small "birth kick". This process may be more common in the universe than previously expected. The direct collapse theory shows advantages in explaining the low kick characteristics of black holes, revealing the diversity of the formation mechanism of stellar-mass black holes.

"More observations in the future will help scientists discover other similar triple black hole systems and provide more support for black hole formation and evolution models. The uniqueness of V404 Cygni not only allows us to re-examine the formation and evolution of black holes, but also brings unprecedented insights into the complexity of black hole systems in the universe."

This article is a work supported by the Science Popularization China Creation Cultivation Program

Author: Gou Lijun

Reviewer: Han Wenbiao, researcher at Shanghai Astronomical Observatory, Chinese Academy of Sciences

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.