# Thin Accretion Disk around a four-dimensional Einstein-Gauss-Bonnet Black Hole

• Recently a novel four-dimensional Einstein-Gauss-Bonnet (4EGB) theory of gravity was proposed by Glavan and Lin [D. Glavan and C. Lin, Phys. Rev. Lett. 124, 081301 (2020)] which includes a regularized Gauss-Bonnet term by using the re-scalaring of the Gauss-Bonnet coupling constant $\alpha \to \alpha/(D-4)$ in the limit $D\to 4$. This theory also has been reformulated to a specific class of the Horndeski theory with an additional scalar degree of freedom and to a spatial covariant version with a Lagrangian multiplier which can eliminate the scalar mode. Here we study the physical properties of the electromagnetic radiation emitted from a thin accretion disk around the static spherically symmetric black hole in the 4EGB gravity. For this purpose, we assume the disk is in a steady-state and in hydrodynamic and thermodynamic equilibrium so that the emitted electromagnetic radiation is a black body spectrum. We study in detail the effects of the Gauss-Bonnet coupling constant $\alpha$ in 4EGB gravity on the energy flux, temperature distribution, and electromagnetic spectrum of the disk. It is shown that with the increases of the parameter $\alpha$, the energy flux, temperature distribution, and electromagnetic spectrum of the accretion disk all increases. Besides, we also show that the accretion efficiency increases as the growth of the parameter $\alpha$. Our results indicate that the thin accretion disk around the static spherically symmetric black hole in the 4EGB gravity is hotter, more luminosity, and more efficient than that around a Schwarzschild black hole with the same mass for a positive $\alpha$, while it is cooler, less luminosity, and less efficient for a negative $\alpha$.

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Cheng Liu, Tao Zhu and Qiang Wu. Thin Accretion Disk around a four-dimensional Einstein-Gauss-Bonnet Black Hole[J]. Chinese Physics C.
Cheng Liu, Tao Zhu and Qiang Wu. Thin Accretion Disk around a four-dimensional Einstein-Gauss-Bonnet Black Hole[J]. Chinese Physics C.
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沈阳化工大学材料科学与工程学院 沈阳 110142

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## Thin Accretion Disk around a four-dimensional Einstein-Gauss-Bonnet Black Hole

###### Corresponding author: Tao Zhu, zhut05@zjut.edu.cn
• a. Institute for Theoretical Physics & Cosmology, Zhejiang University of Technology, Hangzhou, 310023, China
• b. United Center for Gravitational Wave Physics (UCGWP), Zhejiang University of Technology, Hangzhou, 310023, China

Abstract: Recently a novel four-dimensional Einstein-Gauss-Bonnet (4EGB) theory of gravity was proposed by Glavan and Lin [D. Glavan and C. Lin, Phys. Rev. Lett. 124, 081301 (2020)] which includes a regularized Gauss-Bonnet term by using the re-scalaring of the Gauss-Bonnet coupling constant $\alpha \to \alpha/(D-4)$ in the limit $D\to 4$. This theory also has been reformulated to a specific class of the Horndeski theory with an additional scalar degree of freedom and to a spatial covariant version with a Lagrangian multiplier which can eliminate the scalar mode. Here we study the physical properties of the electromagnetic radiation emitted from a thin accretion disk around the static spherically symmetric black hole in the 4EGB gravity. For this purpose, we assume the disk is in a steady-state and in hydrodynamic and thermodynamic equilibrium so that the emitted electromagnetic radiation is a black body spectrum. We study in detail the effects of the Gauss-Bonnet coupling constant $\alpha$ in 4EGB gravity on the energy flux, temperature distribution, and electromagnetic spectrum of the disk. It is shown that with the increases of the parameter $\alpha$, the energy flux, temperature distribution, and electromagnetic spectrum of the accretion disk all increases. Besides, we also show that the accretion efficiency increases as the growth of the parameter $\alpha$. Our results indicate that the thin accretion disk around the static spherically symmetric black hole in the 4EGB gravity is hotter, more luminosity, and more efficient than that around a Schwarzschild black hole with the same mass for a positive $\alpha$, while it is cooler, less luminosity, and less efficient for a negative $\alpha$.

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