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

TL;DR

This paper presents polarized emission observations of M87's black hole with the EHT, revealing magnetic field structures and plasma properties near the event horizon, and compares them with GRMHD simulations to identify consistent models.

Contribution

It provides the first detailed polarization imaging of M87's black hole environment and links observations with magnetically arrested disk models.

Findings

Polarization is scrambled on small scales, likely due to internal Faraday rotation.

Estimated plasma parameters include electron density ~10^4-7 cm^-3, magnetic field ~1-30 G.

Identified GRMHD models with magnetically arrested disks that match observations and support jet formation.

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 of order 10^4-7 cm-3, magnetic field strength B of order 1-30 G, and electron temperature Te of order (1-12) x 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 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) x 10^-4 Msun yr-1.