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

　　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.