Polarization Signatures of Magnetic Reconnection Bubbles near Black Holes

Abstract

Motivated by the observed horizon-scale polarization variability in multi-epoch Event Horizon Telescope (EHT) data, we develop a time-dependent general relativistic polarized ray-tracing framework to study how localized magnetic reconnection near a black hole imprints itself on synchrotron polarization. We model the reconnection phenomenologically as a transient magnetic bubble. While keeping the underlying spacetime, emission prescription, and polarization transport scheme unchanged, the bubble weakens the local ordered field, reorganizes its geometry, induces a conservative local kinematic response, and relaxes through a finite magnetic-memory prescription. We explore how the key bubble parameters control the recovery of the magnetic-field strength, the spatial spreading of the disturbed region, the persistence of the post-reconnection imprint, and the resulting polarization signatures. Perturbations dominated by different magnetic-field components produce distinct polarized morphologies, EVPA evolution, and Stokes-(Q, U) trajectories: azimuthal perturbations favor outward EVPA shifts, radial perturbations favor inward shifts, and vertical perturbations mainly suppress the polarized intensity. For the illustrative M87^*-like cases, the phase evolution of the β_2 polarization coefficient further traces how localized magnetic reconfiguration contributes to the global EVPA twist of the ring. These results show that horizon-scale polarization variability can provide a sensitive diagnostic of localized magnetic restructuring and that the present framework offers a physically transparent way to interpret time-dependent polarimetric signatures in future EHT observations.

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