The Polarized Image of a Synchrotron Emitting Ring of Gas Orbiting a Black Hole

Abstract

Synchrotron radiation from hot gas near a black hole results in a polarized image. The image polarization is determined by effects including the orientation of the magnetic field in the emitting region, relativistic motion of the gas, strong gravitational lensing by the black hole, and parallel transport in the curved spacetime. We explore these effects using a simple model of an axisymmetric, equatorial accretion disk around a Schwarzschild black hole. By using an approximate expression for the null geodesics derived by Beloborodov (2002) and conservation of the Walker-Penrose constant, we provide analytic estimates for the image polarization. We test this model using currently favored general relativistic magnetohydrodynamic simulations of M87*, using ring parameters given by the simulations. For a subset of these with modest Faraday effects, we show that the ring model broadly reproduces the polarimetric image morphology. Our model also predicts the polarization evolution for compact flaring regions, such as those observed from Sgr A* with GRAVITY. With suitably chosen parameters, our simple model can reproduce the EVPA pattern and relative polarized intensity in Event Horizon Telescope images of M87*. Under the physically motivated assumption that the magnetic field trails the fluid velocity, this comparison is consistent with the clockwise rotation inferred from total intensity images.

Image of Bonnor black dihole with a thin accretion disk and its polarization information

“…For a Bonnor dihole (1), it carries a magnetic field itself related to the electromagnetic tensor (6), which plays an important role on the polarized image of the accretion disk around the dihole. In order to study the polarized image of dihole, we must first get the polarization vector of photon emitted by fluid in the disk [15,16]. In the local Cartesian frame {e t, e r , e θ , e φ} of the arbitrary point P in the disk (P -frame ), the electromagnetic tensor F µν (6) can be re-expressed as…”

“…Following [15], the intensity I of linearly polarized light at the observer from the emitter can also be expressed as…”

“…It is because the brightness of the surrounding emission region and the polarization pattern in the image carry a wealth of information about the electromagnetic emissions near the black hole. Thus, a lot of effort has been devoted to making theoretical research on the polarized images of black holes [15,16]. Recently, with a simple ring model, the polarized images of axisymmetric fluid orbiting arising from synchrotron emission in various magnetic field have been investigated for Schwarzschild black hole [15] and Kerr black hole [16].…”

“…Thus, a lot of effort has been devoted to making theoretical research on the polarized images of black holes [15,16]. Recently, with a simple ring model, the polarized images of axisymmetric fluid orbiting arising from synchrotron emission in various magnetic field have been investigated for Schwarzschild black hole [15] and Kerr black hole [16]. Although only the emission within a narrow range of radii R is considered in this simple model, it clearly reveals that the polarization signatures are dominated by magnetic field configuration, black hole spin and observer inclination.…”

Shadow of black hole surrounded by magnetized plasma: Axion-plasmon cloud

“…For instance, various works done entitled the role of non-magnetic plasma on the BH shadow [71][72][73][74][75][76][77][78][79] and gravitational lensing [80][81][82][83][84][85][86], as two famous classical tests of GR. Nevertheless, in the light of the pattern recorded of the magnetic field (MF) and electron temperature within the plasma close to the supermassive M87* BH by the Event Horizon Telescope (EHT) [87,88], it expects that the plasma around BH is magnetized. In other words, these worth observational achievements include this message is that the physics of supermassive BHs, have close relation with the plasma and MF their around.…”