This "black hole observer" gazes into the darkness in the starry sky

There are many evidences to prove that we and the ancients are looking up at the same starry sky. Whether it is the clear and spacious "Tonight is the best night, no one is here to share, I lie down and watch the stars, my mind is clear", or the sad and melancholy "The bells and drums are ringing at the beginning of the long night, and the stars are about to dawn", it shows that humans seem to have become accustomed to placing their emotions on the dense stars above their heads. Regarding this yearning for the universe that is deeply buried in our bones, American writer Timothy Ferries once expressed his unique insights in his works: "In a sense, we are created by stellar matter. It exists in our genes, so it can also be said that this is a natural curiosity." Once upon a time, Wu Jianfeng, a professor in the Department of Astronomy, School of Physical Science and Technology, Xiamen University, was also such a "child who counts stars" with a strong curiosity. He said frankly that he was lucky to be born in an era of rapid technological development. The development of stargazing technology and equipment has undoubtedly built an invisible bridge between him and the universe he longs for.

▲Wu Jianfeng

From visible light to the entire electromagnetic band, to multi-messenger observations such as gravitational waves, neutrinos, and cosmic rays, from night-time observations to rational analysis of the origin of the universe, mankind has gone through a long period of exploration, with many milestone moments and great achievements, but no astronomer can take the honor of discovering a black hole. This celestial body with a space-time curvature so large that even light cannot escape has continued to attract Wu Jianfeng's attention for more than 20 years. "What does a black hole look like? Where does it come from? Where will it end up?" He used these three eternal propositions of Western philosophy to question the countless mysterious stars in the night sky over and over again.

The stars and the moon are bright, and the bright river is in my heart

Wu Jianfeng's first encounter with astrophysics can be traced back to the classroom of Tsinghua University. In 1999, he entered Tsinghua University to study basic science as an undergraduate, and at the same time, he began a new dream-chasing chapter in his life. Physics, which studies the boundless space and time and all the matter in it, is regarded by Wu Jianfeng as the "brilliance of rational thinking" and is worth pursuing with all his heart and soul. Therefore, the "two-point one-line" life trajectory of frequent trips between the library and the classroom did not make him feel boring or annoyed. On the contrary, the contact and research of astrophysics always brought back his childhood memories of gazing at the stars, and strengthened his belief in lifelong pursuit. It was also driven by this interest and belief that has been preserved to this day that the decision to continue to pursue a doctorate in astronomy and astrophysics became inevitable.

After formally completing the intersection of astronomy and physics and becoming an astronomical researcher with a good foundation in science, Wu Jianfeng discovered that each star actually has its own unique past. The combined effects of natural laws and historical accidents have shaped their fate. Humans have tried their best in the long river of history, but have only picked up some fragments of the truth. Therefore, there is still a long way to go before discovering new stars and understanding the universe. Under the bright light of the stars and the moon, there is still a lot to do in the vast world. So while studying for his doctorate, he set his sights on the special celestial bodies called "quasars" by humans.

Quasars, along with pulsars, microwave background radiation and interstellar organic molecules, are known as the "four great discoveries" of astronomy in the 1960s. In essence, although they are named because they look like "star-like objects", they are actually a type of active galactic nucleus, which is the external manifestation of the active accretion state of the supermassive black hole at the center of the galaxy. The mass of the central black hole is often more than a million times that of the sun. They grow by violently swallowing the surrounding gas, which forms a disk-like structure around the black hole, called an accretion disk. Generally speaking, quasars should be a type of dazzlingly bright celestial body - Type I quasars usually have strong broad emission lines in the ultraviolet and optical bands of the spectrum, but in the past 20 years, hundreds of special quasars have been discovered whose broad emission lines are very weak, and the intensity of the emission lines is even more than an order of magnitude lower than that of typical quasars. For a long time, the cause of this weak emission line has always puzzled astronomers.

Wu Jianfeng, a doctoral student, gave his own solution to this problem. He innovatively used a multi-band analysis method and found that the average X-ray radiation intensity of weak emission line quasars was also significantly weaker than that of typical quasars, and thus established a correlation between their special emission line properties and special X-ray properties. Subsequently, Wu Jianfeng and his collaborators hit it off and further proposed a "shielding gas" model, which successfully explained the special emission line properties and X-ray properties of weak emission line quasars. This model has received widespread attention and citations from international peers. Subsequent studies have also found that weak emission line quasars are likely to represent a state in which supermassive black holes accrete surrounding gas at an extremely fast rate (beyond the "Eddington limit"). In other words, the speed at which black holes in this type of quasar devour gas is much faster than that of ordinary quasars. In the earliest stages of the universe, the proportion of this special quasar was significantly higher than that of later periods. Therefore, the study of weak emission line quasars also happened to provide clues to reveal the growth law of the earliest supermassive black holes in the universe, and the conclusions are of great significance. Now, Wu Jianfeng is leading his team to further quantitatively constrain the "shielding gas" model of weak emission-line quasars and their co-evolution laws with their host galaxies from two new perspectives: X-ray light variation and submillimeter wave observations, in order to better expand human understanding of the universe.

Secondly, Wu Jianfeng also did not ignore the study of quasars, the "radio lighthouses" of the early universe. "Radio-noisy quasars" with bright radio radiation have highly collimated relativistic jets, and the high-energy particles they contain stretch over tens of thousands or even millions of light years at speeds close to the speed of light, which is almost one of the most energetic phenomena in the universe. However, although X-ray radiation is an important feature of jets, its generation mechanism has always been a focus of debate in the field. It is generally believed that there are two main radiation mechanisms for jet X-rays - Comptonization of synchrotron radiation itself and inverse Compton scattering of cosmic microwave background radiation, but people still have no consensus on which mechanism plays a leading role. Therefore, Wu Jianfeng studied quasars with high radio noise in the early universe and discovered for the first time the excess X-ray radiation of these quasars, with a relative X-ray radiation intensity of about three times that of similar quasars in the late universe. The evolution of the inverse Compton scattering mechanism of cosmic microwave background radiation over time can well explain this discovery. Although this mechanism does not dominate in the more nearby universe, as the redshift increases, that is, as it moves to the earlier universe, the X-ray radiation produced by this mechanism is significantly enhanced and dominates due to the increase in the energy density of the cosmic microwave background radiation, so the X-ray radiation of the early universe jets will exceed. In short, this work successfully utilized the dominance of this mechanism, linked the history of different periods of the universe, and restored part of the appearance of the early universe for people.

Staring into the "abyss" and "fishing out" the truth

"Every time I work in a new institution, I will try to expand my research field a little bit more." Wu Jianfeng was inspired by his two postdoctoral research experiences at the world's top astronomy research institutions, the Harvard-Smithsonian Center for Astrophysics and the University of Michigan. Thanks to the communication and help of colleagues and the guidance of predecessors, his research field has also expanded from studying black holes to discovering black holes.

▲Wu Jianfeng is teaching "High Energy Astrophysics" to undergraduate students majoring in astronomy at Xiamen University

The first black hole discovered and confirmed in human history is a stellar black hole, which is the product of the end of the life of a massive star. According to the "black hole no-hair theorem", only two basic parameters, mass and spin, are needed to fully describe a black hole in astrophysics (the charge is usually considered to be 0). Of these two parameters, mass is the more basic one. Therefore, accurately measuring the mass of a black hole naturally becomes a prerequisite for other black hole studies. It is certain that the stellar black holes that have been clearly confirmed by electromagnetic band observations today all exist in binary star systems, that is, systems consisting of a black hole and an ordinary star (called a companion star) revolving around each other. Unfortunately, for binary stars whose companion stars are low-mass stars (similar to or smaller than the sun), accurately measuring the mass of their black holes has always been a difficult problem. The main reason for this is that the radiation generated by the accretion disk around the black hole will contaminate the radiation from the companion star, causing the entire system to have additional irregular light variations in the optical band, which will bring significant errors to the measurement of the black hole mass. While doing postdoctoral research at the Harvard-Smithsonian Center for Astrophysics, Wu Jianfeng used rigorous spectral and photometric observation methods for the first time. He determined the radiation proportion of the accretion disk through spectroscopy and used this proportion to correct the light curve obtained by photometry, thereby accurately measuring the mass of the black hole. He also applied this method to the black hole binary star Nova Muscae 1991 (Nova Muscae 1991), correcting the previous deviations in the black hole mass, providing a model for future mass measurements of small-mass black hole binaries.

According to Wu Jianfeng, the black hole binaries that have been discovered and verified in the Milky Way so far only account for the "tip of the iceberg" of the number predicted by theory. "This shows that the traditional method of searching for black hole binaries is inefficient, and there are still many black hole binaries waiting for us to discover." Therefore, at almost the same time, Wu Jianfeng, as one of the core members of the "Galactic Nucleus Survey" project, was committed to exploring new methods for discovering black hole binaries. Its core and purpose was to combine X-ray surveys with subsequent optical spectroscopy observations. Using this method, he and his team quickly selected a group of black hole binary candidates. From yearning for the starry sky to focusing on black holes, Wu Jianfeng has found his own research coordinates in the starry sky - gazing into the abyss of the universe and exploring the truth of the origin.

“The use of uselessness is the greatest use”

Wu Jianfeng has been asked about the "realistic significance of exploring the universe" since his student days. He has been in the School of Physical Science and Technology of Xiamen University for more than 6 years since returning to China. His years of research experience have enabled him to gain deeper wisdom and industry knowledge. His answer to the above question has gradually evolved from "following my interest and satisfying my own curiosity" to eight words - "the use of uselessness is the greatest use".

Is astronomy useless? Does the existence of black holes have nothing to do with our lives? Of course not. This seemingly distant and ethereal natural phenomenon often breeds unknown science and constantly expands the boundaries of human spirit and imagination. The most typical example is the birth of quantum mechanics. It is precisely because of this realization that Wu Jianfeng said that he will remain committed to this "star-chasing" exploration path in the future. Now, Wu Jianfeng has led the team to expand the research to the emerging frontier of "intermediate-mass black holes". The goal is to find and identify new types of black holes with masses between the aforementioned stellar black holes and supermassive black holes, and strive to gain a more complete understanding of the nature and origin of the entire black hole population.

In the vast amount of research work, Wu Jianfeng gradually formed his own research expertise: large-scale sky surveys and multi-band analysis. Large-scale sky surveys represent the development trend of astronomy today, which has truly enabled astronomy to move from an era of scarce data to an era of abundant data. Multi-band analysis is another important scientific research method in contemporary astronomy. "In the past, the information about celestial bodies obtained through a single band was often one-sided, like a blind man touching an elephant. Only by combining data from multiple bands and analyzing the correlation between them can we obtain more comprehensive information," he explained.

As a teacher, Wu Jianfeng also feels that he has the mission to bring astronomy into the vision of more people. Therefore, he not only focuses on developing his own scientific research ability, but also includes the cultivation of students' astronomical literacy in his own requirements. After joining Xiamen University, he paid great attention to course teaching and invested a lot of time and energy to improve his teaching level. His efforts paid off. He also won the first prize in the Xiamen University English Teaching Competition (first place in the science, engineering and medicine group), and the two undergraduate courses he taught were widely praised. Among them, the general course "Black Holes and Gravitational Waves" was also selected as a first-class undergraduate course of Xiamen University, and was invited to become the first batch of first-class courses of the Center for Aesthetic Education and General Education of Xiamen University. "I remember that once I came back from a business trip and was waiting for my luggage at the airport. A student took the initiative to come up and say hello and said that he had taken my "Black Holes and Gravitational Waves" course before. I was very happy." Whenever he mentioned this experience, his eyes were always bright. As the saying goes, "A gold cup or a silver cup is not as good as the reputation of students." Wu Jianfeng knows this well, so he has given popular science lectures and set up astronomy lectures in many primary and secondary schools and science and technology venues outside Xiamen University. These activities have received enthusiastic responses without exception. He said: "In this way, astronomy will be truly close to our daily lives."

In addition to teaching and scientific research, public service is also part of Wu Jianfeng's work. In the "International Telescope Time Program" (TAP) jointly formed by the National Astronomical Big Science Center and many universities, Wu Jianfeng has devoted a lot of effort. He has also served as the rotating chairman of the project's scientific committee, presided over the review of telescope observation applications submitted by various universities, and made his own contribution to the smooth implementation of the project. "This project is one of the main ways for domestic astronomical institutes to obtain observation time for internationally advanced telescopes." He said this about the significance of the project's advancement.

"In fact, I do this not only for what it really needs, but also because I 'can't help it' - I believe that the heart and soul of astronomers should be connected to the starry sky." In addition, Wu Jianfeng also claimed that among many types of books, he likes history books the most, "In addition to knowing the facts, we must also know the reasons behind them. The evolution of things can often most intuitively reflect their essence and core." Therefore, he is still as obsessed with restoring the history of the universe as he was at the beginning, because it contains the origin of mankind and will determine the future of the world to a certain extent.