First M87 Event Horizon Telescope Results. VIII. Magnetic Field Structure near The Event Horizon

TLDR

Polarized synchrotron radiation probes magnetic field structure and plasma properties near the M87 black hole. The study compares EHT‑resolved polarization with simultaneous ALMA observations against theoretical model predictions. GRMHD simulations were quantitatively compared with EHT polarimetric data to identify viable models and estimate the accretion rate. EHT imaging shows low fractional linear polarization, implying Faraday rotation, and the observations are best explained by magnetically arrested accretion disks with inferred densities of 10⁴–10⁷ cm⁻³, magnetic fields of 1–30 G, electron temperatures of (1–12) × 10¹⁰ K, and an accretion rate of (3–20) × 10⁻⁴ M⊙ yr⁻¹.

Abstract

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 fields 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 ∼ 104-7 cm-3, magnetic field strength B ∼ 1-30 G, and electron temperature T e ∼ (1-12) 1010 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 fields 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 sufficient power. The consistent GRMHD models are all of magnetically arrested accretion disks, where near-horizon magnetic fields 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 o˙ yr-1.