| 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. |
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