Shadows and Polarization Images of a Four-dimensional Gauss-Bonnet Black Hole Irradiated by a Thick Accretion Disk

Abstract

We adopt a general relativistic ray-tracing approach to study the shadows and polarization images of spherically symmetric Gauss-Bonnet (GB) black holes enveloped by geometrically thick accretion flows. Specifically, we adopt a phenomenological RIAF-like model and an analytical Hou disk model. In the RIAF-like model, increasing the GB coupling parameter $λ$ reduces both the size and brightness of the higher-order image, while increasing $θ$ alters the shape of the higher-order image and obscures the horizon's outline. The main difference between isotropic and anisotropic emission is that the latter produce distortion of the high-order image in the vertical direction, leading to an elliptical morphology. For the Hou disk model, due to specific regions being geometrically thinner with the conical approximation, the high-order images are narrower with the increase in $λ$ than the RIAF model. While increasing $θ$ enhances the brightness of the direct images outside the higher-order images, but hardly changes the size of the higher-order images, which is in sharp contrast to the RIAF model. Meanwhile, the Hou disk produces polarization patterns that trace the brightness configuration and are affected by $λ$ and $θ$, reflecting the intrinsic structure of spacetime. These results illustrate that intensity and polarization in thick-disk models provide probes of GB black holes and near-horizon accretion dynamics.

Figures (10)

• Figure 1: Shadow images of the black hole for the phenomenological model under isotropic radiation. The accretion flow motion mode is infalling motion. From left to right, the parameter $\lambda$ takes values of $0.01,~0.3,~0.6,~0.99$ respectively. From top to bottom, the observation inclination angle $\theta$ takes values of $17^\circ,~50^\circ,~80^\circ$ respectively. The observer distance is fixed at $500M$, the field of view is $2^\circ$, and the observation frequency is $230$ GHz.
• Figure 2: The first row of images shows: the black hole shadow images under the phenomenological model with isotropic radiation. The accretion flow motion mode is infalling motion. From left to right, the observation frequencies are 85 GHz, 230 GHz, and 345 GHz. The second row of images shows: intensity distribution profile plots at different observation frequencies. The left plot is the horizontal intensity distribution profile, and the right plot is the vertical intensity distribution profile. The observer distance is fixed at $500M$, the field of view is $2^\circ$, the observation inclination is $17^\circ$, and $\lambda=0.6$.
• Figure 3: Black hole shadow images for the phenomenological model under anisotropic radiation. The accretion flow motion mode is infalling motion. From left to right, the parameter $\lambda$ takes values $0.01,~0.3,~0.6,~0.99$, respectively. From top to bottom, the observation inclination angle $\theta$ takes values $17^\circ,~50^\circ,~80^\circ$, respectively. The observer distance is fixed at $500M$, the field of view is $2^\circ$, and the observation frequency is $230$ GHz.
• Figure 4: The first row of images shows: the black hole shadow images under anisotropic radiation for the phenomenological model. The accretion flow motion mode is infalling motion. From left to right, the observation frequencies are $85$ GHz, $230$ GHz, and $345$ GHz. The second row of images shows: the intensity distribution profile plots at different observation frequencies. The left plot is the horizontal intensity distribution profile, and the right plot is the vertical intensity distribution profile. The observer distance is fixed at $500M$, the field of view is $2^\circ$, the observation inclination angle is $17^\circ$, and $\lambda=0.6$.
• Figure 5: Black hole shadow images under the Hou disk model. The accretion flow motion mode is infalling motion. From left to right, the parameter $\lambda$ takes values of $0.1$, $0.3$, $0.6$, and $0.9$, respectively. From top to bottom, the observation inclination $\theta$ takes values of $17^\circ$, $50^\circ$, and $80^\circ$, respectively. The observer distance is fixed at $500M$, the field of view is $2^\circ$, and the observation frequency is $230$ GHz.