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

TL;DR

This study uses relativistic ray-tracing to analyze shadows and polarization images of Gauss-Bonnet black holes with thick accretion disks, revealing how parameters influence observable features and aiding in probing black hole spacetime.

Contribution

It introduces a detailed analysis of shadow and polarization images of Gauss-Bonnet black holes with thick disks using phenomenological models, highlighting effects of GB coupling and disk geometry.

Findings

Increasing GB coupling reduces shadow size and brightness.

Disk geometry and emission anisotropy distort shadow shape.

Polarization patterns reflect spacetime structure and disk properties.

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 $\lambda$ reduces both the size and brightness of the higher-order image, while increasing $\theta$ 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 $\lambda$ than the RIAF model. While increasing $\theta$ 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 $\lambda$ and $\theta$, 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.