Signatures of Black Hole Spin and Plasma Acceleration in Jet Polarimetry II: Off-Axis Jets

Abstract

We analyze the polarization of optically thin, stationary, axisymmetric black hole jets at scales of order the light cylinder radius. Our work generalizes the face-on results of Gelles et al. (2025) to arbitrary viewing inclination. Due to a combination of geometry and relativistic aberration, the polarization of the jet is not left-right symmetric, and the degree of asymmetry can shed light on both the viewing angle and the plasma bulk Lorentz factor. We show that there is always a radius in the jet at which the polarization transitions from azimuthal to radial; this radius is different along the spine and limb of the jet. We propose metrics that can be used to constrain the black hole spin, inclination angle, and plasma Lorentz factor from these polarimetric signatures, and we discuss the impact of limb-brightening on these measurements. We anticipate that these polarimetric signatures can be studied with current or forthcoming data in M87, NGC 315, NGC 4261, Centaurus A, Cygnus A, and other systems. Observations of the polarization of the base of the counter-jet in higher inclination sources would provide a particularly promising probe of black hole spin.