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

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.

• The paper reveals that low linear polarization in EHT images indicates significant internal Faraday rotation and organized poloidal magnetic fields.
• It employs GRMHD simulations with MAD models to match observed polarimetric patterns with theoretical predictions, underscoring the dynamics of black hole environments.
• Observational constraints provide estimates of plasma properties and accretion rates, paving the way for future high-frequency (345 GHz) polarimetric studies.

Overview of The Astrophysical Journal Letters Publication on EHT's Observations of M87

This essay provides a detailed examination of the recent analysis published by the Event Horizon Telescope (EHT) Collaboration, focusing on polarized emission around the supermassive black hole in the M87 galaxy at 230 GHz. The study is pivotal in determining the structure of magnetic fields and the plasma dynamics near the black hole. The research harnesses both resolved polarimetric imaging via EHT and complementary unresolved data from the Atacama Large Millimeter/submillimeter Array (ALMA).

Polarimetric Observations and Theoretical Comparisons

The resolved EHT images exhibit low fractional linear polarization, attributed to internal Faraday rotation, which suggests depolarization due to complex plasma interactions on sub-beam scales. Estimated plasma properties derived from the study include an electron density range of $n_e \sim 10^{4-7} \text{cm}^{-3}$, magnetic field strengths between 1 to 30 G, and an electron temperature of $T_e \sim (1-12) \times 10^{10} \text{K}$. These findings align with the theoretical framework of synchrotron radiation, leading to a significant inference about the azimuthal linear polarization pattern originating from organized poloidal magnetic fields.

GRMHD Simulations and Their Implications

A crucial aspect of the study is its comparative analysis using a substantial library of polarimetric images derived from general relativistic magnetohydrodynamic (GRMHD) simulations. A subset of models that feature magnetically arrested accretion disks (MADs) demonstrates compatibility with observed polarimetric structures and suggests dynamically significant magnetic fields near the event horizon. This research estimates a mass accretion rate onto M87's black hole in the range $(3-20) \times 10^{-4} \text{M}_\odot/\text{yr}$, which presents a moderate efficiency relative to the theoretically ideal thin disk models.

Observational Constraints and Future Directions

Observational constraints in the study are significantly refined via polarimetry, setting bounds on the net linear and circular polarization fractions, along with the azimuthal polarization structure metrics. The results indicate a preference for MAD models, which maintain high accretion events and jet-launching dynamics, as opposed to typical SANE models under comparable conditions.

Moreover, this essay foresees advancements in polarimetric measurements at higher frequencies, such as 345 GHz, expected in future EHT campaigns. Such observations will be pivotal in discriminating between Faraday effects and intrinsic magnetic structures, potentially validating or refuting current magnetic field models.

In conclusion, this analysis by the EHT Collaboration not only solidifies the role of magnetic fields in organizing plasma around the M87 black hole but also paves the way for comprehensive interpretations of black hole environments, merging detailed polarimetric observations with robust theoretical models. The study's findings also provide a launchpad for subsequent research aimed at unraveling the intricacies of relativistic jet initiation and accretion processes in supermassive black hole systems.