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 ∼ 10 4-7 cm −3 , magnetic field 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 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 e yr −1 . Unified AstronomyThesaurus concepts: Accretion (14); Black holes (162); Event horizons (479); Jets (870); Kerr black holes (886); Magnetic fields (994); Magnetohydrodynamics (1964); Plasma astrophysics (1261); Polarimetry (1278); Radiative transfer (1335); Radio jets (1347); Relativistic jets (1390)

The Polarized Image of a Synchrotron-emitting Ring of Gas Orbiting a Black Hole

“…The Event Horizon Telescope (EHT) Collaboration has recently published the first images of a black hole (Event Horizon Telescope Collaboration et al 2019a, 2019b, 2019c, 2019d, 2019e, 2019f, 2021a, 2021b, hereafter EHTC I-VIII, respectively). These images achieve a diffractionlimited angular resolution that corresponds to approximately 5GM/c 2 , where M is the mass of the black hole.…”

Are GRMHD Mean-Field Dynamo Models of Thick Accretion Disks SANE?

“…The asymmetric ring image of the M87 galaxy's compact radio source was reproduced by ray tracing codes in the curved spacetime of the black hole region, using the emission properties of the extremely hot plasma arising from GRMHD simulations [1]. The further processing of polarized emission on the event horizon scale has allowed an even more accurate estimation of the fundamental physical properties of the accreting plasma, such as density, magnetic field intensity, and electron temperature [2].…”

Broadband Multi-wavelength Properties of M87 during the 2017 Event Horizon Telescope Campaign

“…The fact that GRMHD simulations of large-scale jets can self-consistently obtain such a profile further supports this argument (e.g., Chatterjee et al 2019, Figure 13). Upcoming EHTC results on the polarization properties of M87 will provide even more stringent constraints (EHT Collaboration 2021a, 2021b.…”