Achromatic, spin-odd Kerr EVPA as a null Frenet-Serret torsion integral on the photon ring

Abstract

We compute the achromatic gravitational imprint that Kerr spacetime leaves on linear polarization at the photon ring. Recasting parallel transport in a null Frenet--Serret frame yields a single scalar evolution law for the electric-vector position angle. On the observer's screen, the Kerr-minus-Schwarzschild pattern on the direct critical curve is nonzero, strictly odd under spin reversal after a half-turn azimuth relabelling, and tightly confined to a thin annulus. Using backward-shot, Carter-separated geodesics with midpoint RK2 transport, we achieve second-order convergence and degree-scale amplitudes that grow monotonically with spin and inclination (RMS $\simeq 0.5$--$2^{\circ }$ for $a/M\gtrsim 0.8$, $i\gtrsim 60^{\circ }$). Three independent constructions -- Frenet--Serret line integral, explicit Levi--Civita transport of the polarization vector, and the phase of the Walker--Penrose constant -- agree ray by ray. We then define a parity-odd ring estimator that is intrinsically achromatic after standard wavelength-squared regression, symmetry-protected against common even-parity systematics, and compressed into low azimuthal modes. This yields a minimal two-parameter template (spin and inclination) for mm/sub-mm polarimetry of horizon-scale rings in sources such as M87${}^{\ast }$ and Sgr~A${}^{\ast }$. The pipeline enables either a detection of the strong-field parallel-transport phase induced by frame dragging or informative upper limits.

20 pages, 5 figures

References in corpus (7)

• First M87 Event Horizon Telescope Results. II. Array and Instrumentation
• First M87 Event Horizon Telescope Results. VIII. Magnetic Field Structure near The Event Horizon
• Black Hole Shadows, Photon Rings, and Lensing Rings
• First M87 Event Horizon Telescope Results. VII. Polarization of the Ring
• `Hidden' Symmetries of Higher Dimensional Rotating Black Holes
• Optical Polarization Holonomy in the Kerr Metric
• Gravitational Faraday Holonomy