Scientists find 'impossible' black hole in our galaxy with mass 70 times that of the sun: 'Such bodies should not even exist'

Until now, scientists had estimated the mass of an individual stellar black hole in the Milky Way at no more than 20 times that of the sun


                            Scientists find 'impossible' black hole in our galaxy with mass 70 times that of the sun: 'Such bodies should not even exist'

Astronomers have discovered a 'monster' stellar black hole in the Milky Way, many times bigger than ever thought possible.

The research team spotted a stellar black hole with a mass 70 times greater than the sun. Named LB-1, the black hole is located 15 thousand light-years from Earth.

The discovery has come as a massive surprise, researchers said, as until now it was estimated that the mass of an individual stellar black hole in our galaxy would be no more than 20 times that of the sun.

"Black holes of such mass should not even exist in our galaxy, according to most of the current models of stellar evolution," says Professor Jifeng Liu from the National Astronomical Observatory of China of the Chinese Academy of Sciences (NAOC).

"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," says Liu, who led the team.

The Milky Way galaxy is estimated to contain 100 million stellar black holes—cosmic bodies formed by the collapse of massive stars that are so dense that even light cannot escape. 

The finding, made possible by surveying the sky with the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) in China, has been published in Nature.

The discovery, say experts, will force scientists to take a relook at how such objects are formed. "This discovery forces us to re-examine our models of how stellar-mass black holes form," says Laser Interferometer Gravitational-Wave Observatory (LIGO) director Professor David Reitze.

"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," says Professor Reitze from the University of Florida, US. 

An artist’s impression of accretion of gas onto a stellar black hole from its blue companion star. (YU Jingchuan, Beijing Planetarium, 2019)

Until recently, stellar black holes could only be discovered when they gobbled up gas from a companion star. This process, explain experts, creates powerful X-ray emissions, detectable from Earth, that reveal the presence of the collapsed object. However, the vast majority of stellar black holes in our galaxy are not eating up gas from a nearby star, and thus do not emit revealing X-rays. As a result, scientists have been able to identify and measure only about two dozen galactic stellar black holes.

To address this problem, the research team used LAMOST to look for stars that orbit an invisible object, pulled by its gravity. This observational technique was first proposed by an English scientist John Michell in 1783. But it has only become feasible with recent technological improvements in telescopes and detectors, say the researchers.

After the initial discovery using this technique, the researchers used the world's largest optical telescopes—Spain's 10.4-m Gran Telescopio Canarias and the 10-m Keck I telescope in the US—to further determine the system's "physical parameters." The results were phenomenal as the scientists detected a star—eight times heavier than the sun—which was seen orbiting a 70-solar-mass black hole every 79 days.

"All stellar-mass black holes have hitherto been identified by X-rays emitted from gas that is accreting onto the black hole from a companion star. These systems are all binaries with a black-hole mass that is less than 30 times that of the sun. Theory predicts, however, that X-ray-emitting systems form a minority of the total population of star–black-hole binaries. When the black hole is not accreting gas, it can be found through radial-velocity measurements of the motion of the companion star. Here we report radial-velocity measurements taken over two years of the Galactic B-type star, LB-1," the findings state.

It says, “The long orbital period of 78.9 days shows that this is a wide binary system.”

The discovery of LB-1 fits well with another breakthrough in astrophysics. Recently, the LIGO and Virgo gravitational wave detectors have started to catch “ripples in spacetime” caused by collisions of black holes in distant galaxies. The black holes involved in such collisions are also much bigger than what was previously considered typical.

According to the researchers, the direct sighting of LB-1 proves that this population of over-massive stellar black holes exists even in our backyard.

“The LIGO/Virgo experiments have revealed black holes with masses of several tens of solar masses, much higher than previously known Galactic black holes. The discovery of a 70 mass black hole in LB-1 would confirm their existence in our Milky Way,” says the team.

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