Polarization Signatures of Jet Formation on Horizon Scales

Extragalactic jets seen throughout the universe transport energy from small scales near a galaxy’s central supermassive black hole to large distances outside the galaxy itself. These jets may be launched via the Blandford-Znajek mechanism, where magnetic fields around extract a black hole’s spin energy. Polarized synchrotron radiation from M87* observed by the Event Horizon Telescope (EHT) at millimeter wavelengths probes the plasma properties and the structure of magnetic fields near the black hole.In a large library of simulated images from general relativistic magnetohydrodynamic (GRMHD) simulations, the only models consistent with both EHT polarized images and the observed jet power from M87* are magnetically arrested accretion disks, where near-horizon magnetic fields are coherent and dynamically important.Recent work constraining the resolved circular polarization around M87* confirms that the accretion disk is most likely magnetically arrested. The pattern of linear polarization in black hole images directly probes the direction of electromagnetic energy flux; EHT images show that electromagnetic energy flows outward on horizon scales around M87*. Future EHT observations of M87 will be sensitive enough to detect fainter emission both closer to the event horizon and farther downstream in the jet launching region.These observations should enable a definitive test of the Blandford-Znajek mechanism for powering extragalactic jets.