On November 28, 2019, Nature, one of the world’s leading scientific journals, published a major finding led by our astronomers online. The team led by Liu Jifeng and Zhang Wei, researchers at the National Observatory of the Chinese Academy of Sciences, discovered one of the largest stellar-level black holes to date, a 70-fold solar-mass supermassive star-level black hole that far exceeds the upper limit of the theoretical prediction, upsetting perceptions of the formation of star-level black holes. It is bound to drive the innovation of stellar evolution and the theory of black hole formation.
Product: Sina Science and Technology”, “Science Everybody”
Wang Song, Assistant Researcher, National Observatory, Chinese Academy of Sciences, focuses on the activity of black hole binary and star
Li Shuang, Engineer, National Observatory, Chinese Academy of Sciences, Director of Publicity, LAMOST Center for Operations and Development
Lu Yujun, A researcher at the National Observatory of the Chinese Academy of Sciences and a professor at the University of the Chinese Academy of Sciences, studies theoretical astrophysics, including black hole physics, gravitational wave astrophysics and galactic cosmology.
Liu Jifeng is deputy director and researcher of the National Observatory of the Chinese Academy of Sciences and executive vice president of the School of Astronomy and Space Sciences of the University of the Chinese Academy of Sciences. Research directions include dense binary stars, stellar activity, etc.
I. Cosmic light absorbers
Hawking wrote in his final book, Ten Questions, that “facts are sometimes more wonderful than fiction, and black holes are the most true expression of this, and they are more wonderful than anything sci-fi writers imagine.” In 1915, Einstein proposed general relativity, and German physicist Carl Schwarzsi oedd out an accurate solution to Einstein’s field equation, foreshadowing the existence of black holes. Since then, mankind has not stopped imagining and exploring this mysterious object.
In 1965, the Cygnus X-1 became the first black hole homologous body to be discovered because of its strong X-ray radiation; in 2015, gravitational waves detected for the first time provided more concrete evidence of the existence of black holes; and in 2019, astronomers captured visual evidence of black holes – the first black holes – using eight observation sites on four continents over a 10-year period. Fangrong, so that this once “invisible and untouchable” strange celestial body has a trace of affinity. What exactly is a black hole and why has generations of astronomers been so fascinated? It doesn’t glow, and the density is very large (compressing a star 10 times the mass of the sun into a sphere the size of a Beijing six-ring, which is equivalent to the density of a black hole), and has a super-attractive appeal, with any thing passing by it, even the fastest light, which can’t escape, a magical object that is a black hole. Therefore, it can be said that the black hole is a veritable cosmic vacuum “light absorber”.
Astronomers have divided black holes into star-level black holes (100 times below the mass of the sun), medium-mass black holes (100-100,000 times solar mass) and supermassive black holes (100,000 times solar mass) depending on the mass of black holes. Star-level black holes are formed by the death of large-mass stars and are the “inhabitants” of the universe. A star evolves to the end if the remaining mass is too much (more than 3 times the mass of the sun), so that it can neither form a white dwarf nor become a neutron star, once it enters the stage of death, there is no force to prevent the star from collapsing under the ultimate gravitational pull, eventually forming a dense black hole. The center of globular clusters and dwarf galaxies may have medium-mass black holes, while supermassive black holes exist at the center of galaxies, such as the center of the Milky Way, which has a supermassive black hole about 4 million times the mass of the sun.
II. How to observe star-level black holes
Black holes are mysterious and interesting, if dragons dive into the abyss, hide their teeth, and sneak in the universe of stars and seas. If a black hole and a normal star form a close-up binary system, the black hole will reveal its claws, with a powerful “taste” directly to the star’s companion gas matter, forming a accretion disk, emit bright X-ray light (Figure I). These X-ray rays, like the “back-light” before these materials were swallowed by black holes, are powerful clues to astronomers’ search for black holes over the years. Astronomers then measure the mass of a black hole by monitoring the motion of the companion star, which is suitable for the black hole system of the bright companion star. Another approach is for rare double black holes, where scientists listen to the ripples of space-time through gravitational wave experiments, and thus to infer black holes in combination.
Until now, almost all star-level black holes in the Milky Way have been identified by X-rays from the gas of the black hole’s accretion companion star. In the past 50 years, about 20 black holes have been discovered using this method, all between three and 20 times the mass of the sun.
There are hundreds of billions of stars in the Milky Way, and the theory predicts that there should be hundreds of millions of star-level black holes formed by mass deaths in the Milky Way, compared with only a fraction of the amount of X-ray radiation emitted in the black hole binary system. When the black hole and its companion star are far away, our “big stomach king” also shows a calm and gentle side, so how do we search for these flat (non-accliers) black holes? Astronomers have come up with a whole new answer in the discovery of the largest star-level black hole.
Figure 1 An artistic imagination of black hole accretion spewing X-rays (from the web)
Capture the black hole of “hidden”
The national observatory-led research team has discovered an unusually well-behaved binary system in the vast sea of stars, which will contain a hidden black hole? More than 700 days of chasing the road full of hardships and wonderful.
In early 2016, researcher Zhang Wei, director of LAMOST’s Science Survey, and Han Zhanwen, director of the Yunnan Observatory, proposed using LAMOST to observe the dual-star spectrum to carry out a two-star system research program, and selected more than 3,000 objects in Kepler’s Sky Zone (K2-0) for a two-year spectral monitoring. Among them is a “walking pull” b star that has attracted the attention of researchers, which exhibits regular periodic movements and unusual spectral characteristics.
This LAMOST “eye” of the B-star spectrum carries a very rich amount of information, in addition to its effective temperature, surface gravity, metal abundance and other important information, the spectrum of a near-static direction and B-star inverted line (H alpha emission line) to add enough mystery to the star. The researchers suspect there must be a story behind the B-star, which is moving around the invisible “who” movement? It’s really a black hole! Astronomers never easily let any of them down in their pursuit of the truth about the universe.
To further verify the truth behind the special B star, the researchers then applied for 21 observations by the 10.4-meter Canary Large Telescope (GTC) in Spain and seven high-resolution observations by the 10 M.K. Telescope (Keck) in the United States, further confirming the nature of the Type B star.
Figure II Motion And Velocity Curves of Type B Stars and Black Holes in LB-1 Systems
Based on spectral information, the researchers calculated that the metal abundance of the B star was about 1.2 times that of the sun, and that it had a mass of about 8 times that of the sun, and was about 35 million years old. Based on the ratio of the velocity amplitude of the B-star to the H-alpha emission line, the researchers calculated that there was an invisible object in the binary system that was about 70 times the mass of the sun, which could only be a black hole. The “big BOSS” behind the B star was thus dug out by astronomers, such a result is undoubtedly to make people excited and surprised, but the opportunity is always left to the prepared people, not two years ago the vast sea of stars and seas of “spraying”, there is no today’s “main character” appearance.
To commemorate LAMOST’s contribution to the discovery of the giant star-level black hole, astronomers named the binary system, which contains the black hole, LB-1 (Figure 3). Unlike other known star-level black holes, LB-1 has never been detected in any X-ray observations, and the black hole is far from its companion star (1.5 times the daily distance). Using an observation of the source at the Chandra X-ray Observatory in the United States, the researchers found that the newly discovered black hole was a “champion” of a “calm and gentle” stellar black hole, which is very weak in acarcepic to its companion star.
Figure 3 ART Imagination of LB-1 (Yu Jingchuan)
LB-1 is a binary system of X-ray radiation, and it is not feasible to search for such black holes using conventional X-ray methods. Radial velocity monitoring has long been thought to have allowed the discovery of flat-static black hole binary, as confirmed by the discovery of the largest mass black hole to date.
The pre-life of the black hole “champion”
Since 2015, gravitational wave observation experiments at the U.S. Laser Interferometry Gravitational Wave Observatory (LIGO) and the European Virgo Observatory have found black holes dozens of times the mass of the sun, far greater than previously known star-level black holes in the Milky Way.
The 70-fold solar-mass supermassive black hole, discovered by the researchers, not only reveals that such large-mass star-sized black holes exist in the Milky Way, but also refreshes human perceptions of the upper limit of the mass of star-level black holes (Figure IV).
Liu Jifeng, the first author of the paper, said that the general model suggests that large-mass star-level black holes are mainly formed in low metal abundance (less than 1/5 solar metal abundance) environment, while LB-1 has a B-type star similar to the abundance of solar metal. The current theory of stellar evolution predicts that black holes with a maximum mass of 25 times the mass of the sun can be formed at the abundance of solar metals, so the mass of the black hole in LB-1 has entered the “no-go zone” of the existing theory of stellar evolution. This could mean that theories about the evolution of stars into black holes will be forced to rewrite, or that some mechanism of black hole formation has previously been ignored. LIGO station director David. “The discovery of a 70-fold solar-mass black hole in the Milky Way will force astronomers to rewrite models of star-mass black holes,” Reitz commented. This extraordinary achievement, together with the combination of double black holes detected by LIGO and Virgo over the past four years, will contribute to the revival of astrophysical research in black holes.”
Figure 4 Mass Distribution of Black Holes Discovered by The Gravitational Wave Convergence Event and X-ray Method
Another possibility, the black hole in LB-1 may not have been formed by the collapse of a star. The researchers speculate that LB-1 was originally a Three-Body Problem system, and the observed Type B star was in its outermost orbit and was the smallest component of mass, while the current black hole was made up of double black holes formed by the original inner binary. In this case, the system would be an excellent candidate for black hole coincident events and provide a unique laboratory for studying the formation of double black holes in the Three-Body Problem system.
V. The mutual achievements of the “King of Spectroscopic” and the “King of the Black Hole”
The discovery of the “King of the Black Hole” is a testament to the powerful spectral acquisition capabilities of the LAMOST telescope. LAMOST has 4,000 eyes (4,000 fibers) and can observe nearly 4,000 celestial bodies at a time. In March 2019, LAMOST released 11.25 million spectra, becoming the world’s first spectral survey of more than 10 million, and has been hailed by astronomers as the “king of the spectrum” with the highest spectral acquisition in the world (Figure V).
Advanced equipment to promote new discoveries, in this study, China’s independent development of LAMOST played an irreplaceable role. Starting in November 2016, to discover and study spectral binary stars, the researchers used LAMOST to conduct 26 observations of more than 3,000 stars in a sky region of Kepler over a two-year period, with a cumulative exposure time of about 40 hours. If you use an ordinary four-meter telescope to look specifically for such a black hole (365 days a year, eight hours a day), it will take 40 years! This fully reflects LAMOST’s superior observational efficiency!
Figure V LAMOST Telescope and The Sky (National Observatory For Photo)
“Doing good must be a good weapon”, LAMOST this “astronomical weapon” helped astronomers discover today’s protagonist “King of black holes”, and the “King of Black Holes” appearance for the “King of the Spectrum” – LAMOST added more wonderful.
The largest stellar-level black hole to date, the first discovered by LAMOST, will mark the dawn of a new era in the search for black holes using LAMOST’s sky-searching advantage. It is believed that the mutual achievements of the “King of Spectroscopic” and the “King of the Black Hole” will become a good word for the astronomical community.
Six, black hole catcher plan
LAMOST is an active reflection Schmidt telescope developed by Chinese scientists. It uses active optical technology to realize the continuous change of mirror surface in observation, different moments are different Schmidt optical system, breaks through the bottleneck of the large caliber and large field of view of astronomical telescope, is the largest large field of view telescope in the world.
In March 2019, the Guo Shoujing Telescope released 11.25 million spectrals in seven years, twice the number of spectra released by other world cruise projects. In the large-scale spectral survey of the Milky Way, LAMOST has for the first time achieved a major breakthrough in sky coverage, sky-survey volume, sampling density and statistical completeness, filling the gap in China’s large-scale astronomical basic data, and providing an excellent and heritage-worthy sample for the systematic research of the Milky Way, especially the silver disk.
Next, the team will follow with the Black Hole Hunter program: using LAMOST to continue to observe the other four sky regions, using a combination of visual velocity monitoring and sky-testing data, nearly a hundred black holes are expected to be discovered in bulk over five years and their mass is measured, gradually uncovering the dark “family” An iconic step in the study of the formation and evolution of black hole members and the distribution of mass, ushering in a new era of mass discovery of black holes.
Q: Why is LB-1 called “King of The Black Hole”?
A: The black hole we discovered this time has 70 solar masses, more than twice the upper limit of the maximum mass allowed by existing theories. So we call it the “King of the Black Hole”.
Q: Is it the largest black hole in the universe?
A: There are two types of black holes, one is a supermassive black hole at the center of a galaxy, and the other is a star-level black hole formed by the death of a star, the largest of the star-level black holes.
Q: How small is the smallest black hole?
A: In the early days of the universe, there were some primitive black holes that could be very small, as small as an atom. But they have been around for a very short time, and they are no longer visible today.
Q: How far is the newly discovered “King of the Black Hole” from us?
A: Our newly discovered black hole is about 14,000 light-years away from the anti-silver heart of the Milky Way.
Q: How did astronomers look for black holes in the past?
A: One is to push a black hole upside down by listening to a double black hole and a combined-driven tremor. The other is to reverse the existence of a black hole by looking at the motion of the black hole’s companion star.
Q: How does LAMOST look for black holes?
A: If you want to find a black hole in thousands of bright objects, you need to look at them all over, and we have this special ability to see thousands of objects at once. We found this black hole in these thousands of objects.
Q: Is a black hole just not coming in?
A: Generally, we think that a black hole is a big stomach king, mainly to swallow matter, but in fact, the black hole and Hawking radiation, Hawking radiation can take the matter in the black hole, resulting in the loss of black hole matter.
Q: How long does a black hole last and die?
A: The mass of a black hole is lost. The early black holes, by now has basically evaporated, the end of life. Of course, the black holes we discovered today, and those supermassive black holes, have a much longer life span than the universe.
Q: Will black holes eat up the universe?
A: We say that the strong gravitational field of a black hole actually means that it is only strong when it is close to, when it is far away, it is actually with a normal star, and not much difference, so in this sense, we do not have to worry, black holes will not eat the universe into.
Q: Will we take a picture of the “King of the Black Hole”?
A: The black hole we discovered this time is a star-level black hole with a very small mass, and its horizonist is very small, compared to the supermass, M87 black hole, which is much smaller than the previous photo, and our current technology has no way to photograph it. If we had to do one, it wouldn’t even be a little bit less than the black hole we’ve seen before. (Answered by: Liu Jifeng, Deputy Director and Researcher of the National Observatory of the Chinese Academy of Sciences, Executive Vice President of the School of Astronomy and Space Sciences, University of the Chinese Academy of Sciences)