Black Hole Catcher Project is online! LAMOST discovers the largest stellar black hole to date

On November 28, 2019, Nature, a top international scientific journal, published online a major discovery led by Chinese astronomers. A research team led by Liu Jifeng and Zhang Haotong from the National Astronomical Observatory of the Chinese Academy of Sciences discovered a stellar-mass black hole with the largest mass to date. This supermassive stellar-mass black hole with a mass of 70 times that of the sun far exceeds the upper limit of theoretical predictions, subverting people’s understanding of the formation of stellar-mass black holes and is bound to promote the innovation of stellar evolution and black hole formation theory.

1. Cosmic Light Absorber

　　Hawking wrote in his last book, Ten Questions, “Facts are sometimes stranger than fiction, and nowhere is this more true than in the case of black holes, which are stranger than anything imagined by science fiction writers.” In 1915, Einstein proposed the theory of general relativity, and German physicist Karl Schwarzschild derived an exact solution to Einstein’s field equations, predicting the existence of black holes. Since then, humans have never stopped imagining and exploring this mysterious celestial body.

　　In 1965, Cygnus X-1 became the first black hole candidate discovered due to its strong X-ray radiation; in 2015, the first detected gravitational waves provided more concrete evidence for the existence of black holes; in 2019, astronomers spent 10 years using eight observation points on four continents to capture visual evidence of a black hole – the first black hole “face”, which made this once “invisible and intangible” strange celestial body a little more approachable. What exactly is a black hole, and why do generations of astronomers fascinate them so much? It does not emit light itself, but has a very high density (compressing a star 10 times the mass of the sun into a sphere with a diameter of the Beijing Six Rings, such a density is equivalent to the density of a black hole), and has a super strong attraction. Any matter passing by it, even the fastest light, cannot escape. This magical celestial body is a black hole. Therefore, it can be said that a black hole is a veritable “light absorber” of the cosmic vacuum.

　　Astronomers roughly divide black holes into stellar black holes (less than 100 times the mass of the sun), intermediate-mass black holes (100 to 100,000 times the mass of the sun) and supermassive black holes (more than 100,000 times the mass of the sun) based on their masses. Stellar black holes are formed by the death of massive stars and are widely distributed “residents” in the universe. If a star has too much mass left (greater than 3 times the mass of the sun) at the end of its evolution, it can neither form a white dwarf nor a neutron star. Once it enters the death stage, there is no force that can prevent the star from continuing to collapse under the ultimate gravity, eventually forming a dense black hole. There may be intermediate-mass black holes in the center of globular clusters and dwarf galaxies, while there are supermassive black holes in the center of galaxies. For example, there is a supermassive black hole with a mass of about 4 million times the mass of the sun at the center of the Milky Way.

2. How to observe stellar black holes

　　Black holes are mysterious and interesting, like a dragon lurking in the abyss, hiding its claws and stalking in the sea of ​​stars. If a black hole forms a close binary system with a normal star, the black hole will reveal its hideous claws and directly suck the gas and matter from the companion star with its powerful “appetite”, forming an accretion disk and emitting bright X-ray light (Figure 1). These X-ray lights are like the “afterglow” before these substances are swallowed by the black hole. It is this “light” that has become a powerful clue for astronomers to track the traces of black holes in the past few years. Then, astronomers will measure the mass of the black hole by monitoring the movement of the companion star, which is applicable to black hole systems with bright companion stars. Another method is that for rare binary black holes, scientists mainly listen to the ripples of space-time through gravitational wave experiments, and then infer black hole merger events.

　　To date, almost all stellar-mass black holes in the Milky Way have been identified through the X-rays emitted by the black holes as they accrete gas from companion stars. In the past fifty years, about twenty black holes have been discovered using this method, with masses ranging from 3 to 20 times the mass of the sun.

　　There are hundreds of billions of stars in the Milky Way. According to theoretical predictions, there should be hundreds of millions of massive dead stellar black holes in the Milky Way. In black hole binary systems, only a small fraction can emit X-ray radiation. When the black hole and its companion star are far apart, our “big eater” will also show a calm and gentle side. So how to search for these static black holes (not accreting companion star gas)? Astronomers have given a brand new answer in the process of discovering this largest stellar black hole.

Figure 1 An artistic conception of a black hole accreting X-rays (from the Internet)

3. Capturing the “hidden” black hole

　　A research team led by the National Astronomical Observatory of China discovered an unusual binary star system in the vast sea of ​​stars. Could it contain a hidden black hole? The 700-day pursuit was full of hardships and excitement.

　　In early 2016, Zhang Haotong, director of the LAMOST Scientific Survey Department, and Han Zhanwen, an academician of the Yunnan Astronomical Observatory, proposed using LAMOST to observe the spectra of binary stars and conduct research on binary star systems. They selected more than 3,000 celestial bodies in a Kepler region (K2-0) for a two-year spectral monitoring. Among them, a B-type star with a “cool walk” attracted the attention of researchers. This star showed regular periodic motion and unusual spectral characteristics.