UvA-DARE (Digital Academic Repository) First M87 Event Horizon Telescope Results. VIII Magnetic Field Structure near The Event Horizon

Event Horizon Telescope ( EHT ) observations at 230 GHz have now imaged polarized emission around the supermassive black hole in M87 on event-horizon scales. This polarized synchrotron radiation probes the structure of magnetic fi elds and the plasma properties near the black hole. Here we compare the resolved polarization structure observed by the EHT, along with simultaneous unresolved observations with the Atacama Large Millimeter / submillimeter Array, to expectations from theoretical models. The low fractional linear polarization in the resolved image suggests that the polarization is scrambled on scales smaller than the EHT beam, which we attribute to Faraday rotation internal to the emission region. We estimate the average density n e ∼ 10 4 – 7 cm − 3 , magnetic fi eld strength B ∼ 1 – 30 G, and electron temperature T e ∼ ( 1 – 12 ) × 10 10 K of the radiating plasma in a simple one-zone emission model. We show that the net azimuthal linear polarization pattern may result from organized, poloidal magnetic fi elds in the emission region. In a quantitative comparison with a large library of simulated polarimetric images from general relativistic magnetohydrodynamic ( GRMHD ) simulations, we identify a subset of physical models that can explain critical features of the polarimetric EHT observations while producing a relativistic jet of suf fi cient power. The consistent GRMHD models are all of magnetically arrested accretion disks, where near-horizon magnetic fi elds are dynamically important. We use the models to infer a mass accretion rate onto the black hole in M87 of ( 3 – 20 ) × 10 − 4 M e yr − 1 . Uni ed Accretion Black 162 Event horizons 479 Jets 870 Kerr