We have studied polarized image of a Schwarzschild black hole with an equatorial thin accretion disk as photon couples to Weyl tensor. The birefringence of photon originating from the coupling affect the black hole shadow, the thin disk pattern and its luminosity distribution. We also analyze the observed polarized intensity in the sky plane. The observed polarized intensity in the bright region is stronger than that in the darker region. The stronger effect of the coupling on the observed polarized vector appears only in the bright region close to black hole. These features in the polarized image could help us to understand black hole shadow, the thin accretion disk and the coupling between photon and Weyl tensor.
We have studied the shadow and its luminosity for a static and regular phantom compact object under the static spherical accretion and the infalling spherical accretion, respectively. Comparing with the usual Schwarzschild black hole, the presence of phantom hair yields the larger black hole shadow and the darker image. In both spherical accretion models, with the increase of phantom parameter, the maximum luminosity occurred at photon ring and the brightness of the central region in the shadow decrease, but in the region far from the shadow, the luminosity of image slightly increases. The image of a phantom wormhole and its luminosity have similar behaviors. Moreover, as the phantom charge parameter α increases up to the critical value at where the compact object changes from black hole to wormhole, there exists a jump for the specific intensity, which also appears in the slowly rotating case. This implies that the phantom hair is imprinted on both the shadow radius and the intensity of the electromagnetic flux radiation around compacted object.
We have studied the polarized image of an equatorial emitting ring around a 4D Gauss-Bonnet black hole. Our results show that the effects of Gauss-Bonnet parameter on the polarized image depends on the magnetic field configuration, the observation inclination angle, and the fluid velocity in the disk. For the case with pure equatorial magnetic field, the observed polarization intensity increases with Gauss-Bonnet parameter as the observation inclination angle is small, but this monotonicity gradually disappears with the increase of the inclination angle. However, in the case where the magnetic field is vertical to the equatorial plane, the polarization intensity is an increasing function of Gauss-Bonnet parameter as the inclination angle is large. The changes of the electric vector position angle (EVPA) with Gauss-Bonnet parameter in both cases are more complicated. We also probe the effects of Gauss-Bonnet parameter on the Strokes Q-U loops.
The polarized images of a synchrotron emitting ring are studied in the spacetime of a rotating black hole in the scalar–tensor–vector–gravity (STVG) theory. The black hole owns an additional dimensionless modified gravity (MOG) parameter described as its deviation from a Kerr black hole. The effects of the MOG parameter on the observed polarization vector and Strokes Q − U loops depend heavily on the spin parameter, the magnetic field configuration, the fluid velocity, and the observation inclination angle. For the fixed MOG parameter, the changes of the polarization vector in the image plane are similar to those in the Kerr black hole case. The comparison of the polarization images between the Kerr−MOG black hole and M87* implies that there remains some possibility for the STVG−MOG theory.
We have studied the polarized image of an equatorial emitting ring around a 4D Gauss–Bonnet black hole. Our results show that the effects of Gauss–Bonnet parameter on the polarized image depend on the magnetic field configuration, the observation inclination angle, and the fluid velocity. As the magnetic field lies in the equatorial plane, the observed polarization intensity increases monotonously with Gauss–Bonnet parameter in the low inclination angle case, and its monotonicity disappears in the case with high inclination angle. However, as the magnetic field is vertical to the equatorial plane, the polarization intensity is a monotonously increasing function of Gauss–Bonnet parameter in the high inclination angle case. The changes of the electric vector position angle with Gauss–Bonnet parameter in both cases are more complicated. We also probe the effects of Gauss–Bonnet parameter on the Strokes Q–U loops.
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