Signatures of Black Hole Spin and Plasma Acceleration in Jet Polarimetry

TL;DR

This paper links polarization swings in black hole jets to key physical scales, especially the light cylinder, proposing a new method to measure black hole spin from polarized jet images.

Contribution

It introduces an analytical approach connecting jet polarization features to black hole spin, enabling spin measurement via polarized interferometric imaging.

Findings

Polarization swings occur at specific jet radii related to black hole properties.

The method predicts observable polarization signatures near the black hole horizon.

Future telescopes can measure black hole spin through jet polarization analysis.

Abstract

We study the polarization of black hole jets on scales of $10-10^3\,GM/c^2$ and show that large spatial swings in the polarization occur at three characteristic distances from the black hole: the radius where the counter-jet dims, the radius where the magnetic field becomes azimuthally dominated (the light cylinder), and the radius where the plasma reaches its terminal Lorentz factor. To demonstrate the existence of these swings, we derive a correspondence between axisymmetric magnetohydrodynamic outflows and their force-free limits, which allows us to analytically compute the plasma kinematics and magnetic field structure of collimated, general relativistic jets. We then use this method to ray trace polarized images of black hole jets with a wide range of physical parameters, focusing on roughly face-on jets like that of M87. We show that the location of the polarization swings is strongly tied to the location of the light cylinder and thus to the black hole's spin, illustrating a new method of measuring spin from polarized images of the jet. This signature of black hole spin should be observable by future interferometric arrays like the (Next Generation) Event Horizon Telescope, which will be able to resolve the polarized emission of the jet down to the near-horizon region at high dynamic range.