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.

　　The spectrum of the B-type star in the “eyes” of LAMOST carries a wealth of information. In addition to important information such as its effective temperature, surface gravity, and metallicity, a bright line (Hα emission line) in the spectrum that is almost stationary and moves in the opposite direction to the B-type star adds enough mystery to the star. Researchers suspect that there must be a story behind this B-type star. What is it orbiting around the invisible “who”? Could it really be a black hole? Astronomers will never let go of any possibility easily in their pursuit of the truth of the universe.

　　To further verify the truth behind this special B-type star, the researchers immediately applied for 21 observations from Spain’s 10.4-meter Gran Telescopio Canarias (GTC) and 7 high-resolution observations from the United States’ 10-meter Keck Telescope (Keck), further confirming the properties of the B-type star.

Figure 2 The motion patterns and velocity curves of B-type stars and black holes in the LB-1 system

　　Based on the spectral information, the researchers calculated that the metal abundance of the B-type star is about 1.2 times that of the sun, the mass is about 8 times that of the sun, and the age is about 35 million years. Based on the velocity amplitude ratio of the B-type star and the Hα emission line, the researchers calculated that there is an invisible celestial body with a mass of about 70 times that of the sun in the binary system, which can only be a black hole. The “big boss” behind the B-type star was thus dug out by astronomers. Such a result undoubtedly makes people excited and surprised. However, opportunities are always reserved for those who are prepared. Without the “casting of nets” in the vast sea of ​​stars two years ago, there would be no appearance of the “protagonist” today.

　　To commemorate LAMOST’s contribution to the discovery of this huge stellar black hole, astronomers named the binary system containing the black hole LB-1 (Figure 3). Unlike other known stellar black holes, LB-1 has never been detected in any X-ray observations. This black hole and its companion star are far apart (1.5 times the distance between the sun and the earth). Researchers used the Chandra X-ray Observatory in the United States to observe the source and found that the newly discovered black hole accreted very weakly on its companion star, making it a “calm and gentle” stellar black hole “champion”.

Figure 3 Artistic conception of LB-1 (drawn by Yu Jingchuan)

　　LB-1 is a binary system with quiet X-ray radiation, and it is not feasible to search for such black holes using conventional X-ray methods. It has long been believed that radial velocity monitoring can detect quiet black hole binaries, and the discovery of the most massive black hole to date confirms this.

4. The past and present of the black hole “champion”

　　Since 2015, gravitational wave observation experiments by the U.S. Laser Interferometer Gravitational-Wave Observatory (LIGO) and the European Virgo Gravitational-Wave Observatory (Virgo) have discovered black holes with masses dozens of times that of the sun, which are much more massive than the previously known stellar-mass black holes in the Milky Way.

　　The supermassive black hole discovered by the researchers this time, which is 70 times the mass of the sun, not only reveals the existence of such massive stellar-mass black holes in the Milky Way, but also refreshes human understanding of the upper limit of the mass of stellar-mass black holes (Figure 4).

　　Researcher Liu Jifeng, the first author of the paper, said that the general model believes that massive stellar black holes are mainly formed in low-metallicity environments (less than 1/5 of the solar metallicity), but LB-1 has a B-type star with a metallicity close to that of the sun. The current stellar evolution theory predicts that only black holes with a maximum mass of 25 times that of the sun can be formed under solar metallicity. Therefore, the mass of the black hole in LB-1 has broken through the “forbidden zone” of the existing stellar evolution theory. This may mean that the theory of black hole formation through stellar evolution will be forced to be rewritten, or that some previous black hole formation mechanisms have been overlooked. LIGO Director David Reitz commented, “The discovery of a black hole with a mass of 70 times that of the sun in the Milky Way will force astronomers to rewrite the formation model of stellar-mass black holes. This extraordinary achievement, together with the binary black hole merger events detected by LIGO and Virgo in the past four years, will promote the revival of black hole astrophysics research.”

Figure 4 LB-1 and gravitational wave merger events, and the mass distribution of black holes discovered by X-ray methods

　　Another possibility is that the black hole in LB-1 may not have been formed by the collapse of a single star. The researchers speculate that LB-1 was originally a three-body system, with the observed B-type star in the outermost orbit being the smallest component, and the current black hole being formed by the merger of two black holes formed by the original inner binary star. In this case, the system would be an excellent candidate for a black hole merger event and provide a unique laboratory for studying the formation of binary black holes in a three-body system.

5. The mutual achievements of the “King of Spectra” and the “King of Black Holes”

　　The discovery of this “king of black holes” fully confirms the powerful spectrum acquisition capability of the LAMOST telescope. LAMOST has 4,000 eyes (4,000 optical fibers) and can observe nearly 4,000 celestial bodies at a time. In March 2019, LAMOST publicly released 11.25 million spectra, becoming the world’s first spectral survey project to exceed 10 million, and was hailed by astronomers as the “king of spectra” with the highest spectrum acquisition rate in the world (Figure 5).

　　Advanced equipment promotes new discoveries. In this study, LAMOST, independently developed by my country, played an irreplaceable role. Starting from November 2016, in order to discover and study spectroscopic binaries, researchers used LAMOST to observe more than 3,000 stars in one area of ​​Kepler’s sky 26 times over two years, with a cumulative exposure time of about 40 hours. If an ordinary four-meter telescope is used to specifically search for such a black hole (observation 365 days a year, 8 hours a day), it will take 40 years with the same probability! This fully reflects the ultra-high observation efficiency of LAMOST!

Figure 5 LAMOST telescope and starry sky (Photo courtesy of the National Astronomical Observatory of China)

　　“If you want to do your work well, you must first sharpen your tools.” LAMOST, this “astronomical tool”, helped astronomers discover today’s protagonist, the “king of black holes”. The appearance of the “king of black holes” also added more excitement to the “king of spectra” – LAMOST.

　　This is the most massive stellar black hole so far and the first black hole discovered by LAMOST. Its appearance will mark the arrival of a new era of searching for black holes using the advantages of LAMOST’s sky survey. I believe that the mutual achievements of the “King of Spectra” and the “King of Black Holes” will become a story that the astronomical community will talk about with great relish.