Last month, Professor Feng Yuan from Shanghai Astronomical Observatory (SHAO) published an important review article on the nature of hot accretion flows of gas around black holes. The article entitled “Hot Accretion Flows around Black Holes” was written with Professor Narayan of Harvard University. This is only the second review article written by a domestic Chinese astronomer to be featured in the top review journal in Astronomy in the world — the “Annual Review of Astronomy and Astrophysics” (ARA&A).
What is black hole accretion? Black holes are the most mysterious while ubiquitous object in the universe. Black hole accretion, as one fundamental physical process in the universe, describes the process that a black hole swallows enormous gas from its surroundings. During the falling of the gas, its gravitational energy is converted into the internal energy thus strong radiation is emitted. The accretion disks that form during the accretion of gas onto these objects power X-ray binaries, active galactic nuclei (AGN) like quasars, and, most likely, gamma ray bursts (GRBs) (the most energetic events in the Universe besides the Big Bang itself!). What’s more, accretionalso plays a crucial role in the fields of star formation and planet formation. Because of the importance and ubiquity of black hole and accretion theory, the Shaw Prize (which is regarded as “Oriental Nobel Prize”) were awarded to three astronomers working on the area of the black hole and accretion in 2008 and 2013. In spite of many achivements, many details of how energy is converted within these systems, powering these phenomena, is not fully understood and remains an active area of research.
Black hole accretion models can be divided into two broad classes — cold or hot — depending on the temperatures of their accretion flows. Cold accretion disks are thought to exist in luminous AGNs since cold flows have high radiative efficiencies — and hence, higher luminosities. The theory of cold accretion disks and its application to understanding AGNs was largely established by the mid-80’s and two review articles were written for the ARA&A in 1981 and 1984 by two professors from the University of Cambridge.In hot accretion flow models, the temperatures of gas flows are about 100,000 times higher than temperatures found in cold accretion flows. In this case the radiation efficiency is lower because most of the released gravitational energy remains stored in the accretion flow as it is advected (that is, carried by the flow) into the black hole rather than being radiated away. This successfully explains why most of the galactic nuclei in the universe are so dim. While the hot accretion flow model was first proposed in the 1970s, its importance was not well recognized until its revival in 1994 by Prof. Narayan from Harvard University, who is also the second author of this review article. He was also elected as a fellow of the Royal Society and the American Academy of Sciences because of his contributions to this field. Since then, astronomers have been carrying out systematic and extensive observational studies of hot accretion flows.
Profs. Yuan started his study of hot accretion flow from the early beginning of this field. In the past fifteen years, he and his collaborators have written many influential papersand made creative contributions. These include the discovery of a new hot accretion solution, the radiatively-inefficient accretion flow model for the supermassive black hole in the Galactic center, the MHD model for the formation of episodic jet, the proof of the existence of significant wind from hot accretion flow, and various applications of the hot accretion flow theory in AGNs and black hole X-ray binaries. Therefore, he was selected by the community and the editorial committee of 《ARA&A》 to be the first author, together with Prof. Narayan, to review our best understanding to-date on black hole hot accretion flows. They include the dynamics of accretion flows, radiation processes, microphysics, jet formation, and applications to various systems such as the supermassive black hole in our Galaxy, AGNs, black hole binaries and AGN feedback.
Prof. Yuan was a member of the CAS One Hundred Talent Program and has been awarded a grant from the National Distinguished Youth Fund. He is the current director of the Research Center for Galaxies and Cosmology at SHAO.
Prof. Narayan from Harvard University is a Fellow of the Royal Society and the American Academy of Sciences.
Feng Yuan, Shanghai Astronomical Observatory, Chinese Academy of Science firstname.lastname@example.org