Black Hole Ringdown Seen in Photon Polarization Swings

TL;DR

This paper develops a covariant framework for polarized photon propagation in curved spacetime, revealing how polarization swings during black hole ringdown encode gravitational wave information.

Contribution

It introduces a new perturbative approach to connect electromagnetic polarization signals with black hole ringdown modes, enabling direct observational probes.

Findings

Photon polarization swings are locked to black hole quasi-normal modes.

Achromatic, damped polarization oscillations track the ringdown phase.

Polarization signatures can reach amplitudes of about 10 degrees.

Abstract

Light propagating through a perturbed spacetime could imprint the underlying gravitational waveform directly onto electromagnetic observables. In this Letter, we develop a covariant perturbative framework for polarized photon propagation in generic curved spacetimes, and derive a compact expression for the observable polarization-angle (PA) swing during Kerr ringdown, explicitly demonstrating its time-domain locking to the quasi-normal modes. We confirm this behavior using dynamical ray-tracing calculations for a broad class of photon trajectories. Photons grazing the strong-field region exhibit an achromatic, damped PA oscillation that tracks the ringdown, with a phase set by the mode's angular structure. The swing amplitude can reach $\sim 10^{\circ}$ and leaves distinctive signatures in spatially resolved autocorrelations. These results open a new polarimetric window onto black hole mergers and ringdown.