12 research outputs found
The polarized image of a synchrotron-emitting ring of gas orbiting a black hole
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 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.http://iopscience.iop.org/0004-637Xam2023Physic