Universal Horizon Polarization Pattern Enables Direct Spin Measurement in M87* via EHT Polarimetry
The work establishes a spin- and inclination-only polarization signature at the horizon that GRMHD simulations approach. It supplies a falsifiable observable for ngEHT to confirm horizon-threading fields and measure M87* spin. The main uncertainty remains whether turbulent fluctuations will average sufficiently at achievable resolutions.
The principal limitation is that the derivation assumes time-stationary, perfectly degenerate magnetospheres; real disks exhibit turbulence and non-zero resistivity that can rotate the polarization vector on orbital timescales. Next-generation Event Horizon Telescope observations with improved uv-coverage and simultaneous multi-frequency coverage will be required to average down these fluctuations below the predicted horizon signal. If the observed radial polarization trend matches the analytic curve to within 10 degrees at 5 microarcsecond resolution, the measurement would constitute the first direct evidence that black-hole spin energy is being tapped electromagnetically.
Chael et al.: ngEHT 230 GHz maps of M87* will show polarization angle twist within 8 degrees of the analytic horizon curve at r<3 r_g once averaged over 10 nights.
Sources (3)
- [1]Primary Source(https://arxiv.org/abs/2606.12518)
- [2]EHT M87* Polarization Paper(https://iopscience.iop.org/article/10.3847/2041-8213/ab0c57)
- [3]Blandford-Znajek Original(https://ui.adsabs.harvard.edu/abs/1977MNRAS.179..433B)