Scientists reveal new laws of accretion radiant energy spectroscopy for supermassive black holes

Associate Professor Cai Zhenyi and Professor Wang Junxian of the Department of Astronomy at the University of Science and Technology of China overturned the traditional understanding of the field by studying the polar ultraviolet radiation energy spectrum accreted by the supermassive black hole at the center of the quasar. They further found that the mean extreme ultraviolet energy spectrum of quasars is much softer than expected by classical accretion disk theory, which poses a serious challenge to the classical accretion disk radiation model and strongly supports the accretion model with universal coil wind. On October 5, the results were published online in Nature Astronomy.

Schematic diagram of a black hole accretion disk. Source: Pixabay

Quasars are a very bright class of extragalactic objects whose central supermassive black hole continuously devours gas in the core region of the galaxy. Huge gravitational potentials are released on the accretion disk formed by gas, converted into heat and electromagnetic radiation, making the galaxy’s core unusually bright. Quasars are also known as the “superbeasts” of the universe because of their ultra-high intrinsic brightness. The standard accretion disk theory shows that the accretion disk produces the famous “big blue packet” radiation signature, and the theoretical expected peak is in the extreme ultraviolet band. The more massive the central black hole, the lower the temperature of the accretion disk and the softer the extreme ultraviolet energy spectrum. Observations have found that brighter quasars (the more massive the black hole) have relatively weaker emission lines (which can be explained by softer extremely ultraviolet energy spectra), the famous Baldwin effect, which also seems to be consistent with the classical accretion disk theory model.

Cai Zhenyi and Wang Junxian’s study directly focused on the optical-extreme ultraviolet energy spectrum of large sample quasars, and the study used ground-based SDSS and space GALEX observation data to control the influence of incomplete polar ultraviolet detection, and found that the mean polar ultraviolet energy spectrum of quasars does not depend on intrinsic brightness, which not only indicates that the difference in intrinsic brightness cannot explain the Baldwin effect, but also significantly challenges the prediction of standard accretion disk theory. At the same time, they give a possible new physical origin of the Baldwin effect: the brighter the quasar, the smaller the thermal fluctuation of its accretion disk, and thus cannot produce more emitting line region clouds.

By correcting for the effects of intergalactic media absorption, the study also found that the quasar’s polar ultraviolet average energy spectrum was softer than all previous studies, presenting a further serious challenge to the standard accretion disk model. This ultra-soft extreme ultraviolet spectrum , independent of intrinsic brightness fits well with the prediction of the accretion disk model with coil winds, indicating the ubiquity of disk winds in quasars.

The results of this study have extensive implications for the understanding of massive black hole accretion physics, black hole mass growth, cosmic reionization, physical origin of wide line regions, extreme ultraviolet dust extinction, etc. In the future, satellite projects with ultraviolet detection capabilities, such as the China Sky Survey Space Telescope, will greatly enhance people’s understanding of the many physical properties of quasars. (Source: Wang Min, China Science News)

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