Figure LB-1: Accretion of gas onto a stellar black hole from its blue companion star, through a truncated accretion disk (Artist impression). (Image by YU Jingchuan, Beijing Planetarium, 2019)

The discovery came as a big surprise. "Black holes of such mass should not even exist in our Galaxy, according to most of the current models of stellar evolution," said Prof. LIU. "We thought that very massive stars with the chemical composition typical of our Galaxy must shed most of their gas in powerful stellar winds, as they approach the end of their life. Therefore, they should not leave behind such a massive remnant. LB-1 is twice as massive as what we thought possible. Now theorists will have to take up the challenge of explaining its formation." 

Until just a few years ago, stellar black holes could only be discovered when they gobbled up gas from a companion star. This process creates powerful X-ray emissions, detectable from Earth, that reveal the presence of the collapsed object. 

The vast majority of stellar black holes in our Galaxy are not engaged in a cosmic banquet, though, and thus don’t emit revealing X-rays. As a result, only about two dozen Galactic stellar black holes have been well identified and measured. 

To counter this limitation, Prof. LIU and collaborators surveyed the sky with China’s Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST), looking for stars that orbit an invisible object, pulled by its gravity.  

This observational technique was first proposed by the visionary English scientist John Michell in 1783, but it has only become feasible with recent technological improvements in telescopes and detectors. 

Still, such a search is like looking for the proverbial needle in a haystack: only one star in a thousand may be circling a black hole. 

After the initial discovery, the world’s largest optical telescopes – Spain’s 10.4-m Gran Telescopio Canarias and the 10-m Keck I telescope in the United States – were used to determine the system’s physical parameters. The results were nothing short of fantastic: a star eight times heavier than the Sun was seen orbiting a 70-solar-mass black hole, every 79 days. 

The discovery of LB-1 fits nicely with another breakthrough in astrophysics. Recently, the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo gravitational wave detectors have begun to catch ripples in spacetime caused by collisions of black holes in distant galaxies. Intriguingly, the black holes involved in such collisions are also much bigger than what was previously considered typical. 

The direct sighting of LB-1 proves that this population of over-massive stellar black holes exists even in our own backyard. "This discovery forces us to re-examine our models of how stellar-mass black holes form," said LIGO Director Prof. David Reitze from the University of Florida in the U.S. 

"This remarkable result along with the LIGO-Virgo detections of binary black hole collisions during the past four years really points towards a renaissance in our understanding of black hole astrophysics," said Reitze. 

This work was made possible by LAMOST (Xinglong, China), the Gran Telescopio Canarias (Canary Islands, Spain), the W. M. Keck Observatory (Hawaii, United States), and the Chandra X-ray Observatory (United States). The research team comprised scientists from China, the United States, Spain, Australia, Italy, Poland and the Netherlands.  

The paper can be found at https://www.nature.com/articles/s41586-019-1766-2.