Abstract
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 ⊙ yr−1.
Highlights
The Event Horizon Telescope (EHT) Collaboration has recently published total intensity images of event-horizon-scale emission around the supermassive black hole in the core of the M87 galaxy (M87*; Event Horizon Telescope Collaboration et al 2019a, 2019b, 2019c, 2019d, hereafter EHTC I, EHTC II, EHTC III, EHTC IV)
The simulation library generated for the analysis of the EHT 2017 total intensity data in EHTC V consists of a set of 3D general relativistic magnetohydrodynamic (GRMHD) simulations that were postprocessed to generate simulated black hole images via general relativistic radiative transfer (GRRT)
In the purely thermal case, Faraday rotation depolarizes the emission at the EHT beam scale, producing low fractional polarization across the image that is inconsistent with EHT observations of M87*
Summary
The Event Horizon Telescope (EHT) Collaboration has recently published total intensity images of event-horizon-scale emission around the supermassive black hole in the core of the M87 galaxy (M87*; Event Horizon Telescope Collaboration et al 2019a, 2019b, 2019c, 2019d, hereafter EHTC I, EHTC II, EHTC III, EHTC IV). The EHT polarimetric measurements are consistent with unresolved observations of the radio core at the same frequency with the Submillimeter Array (SMA; Kuo et al 2014) and the Atacama Large Millimeter/submillimeter Array (ALMA; Goddi et al 2021) They provide a detailed view of the polarized emission region on event-horizon scales near the black hole. Configuration and magnetized plasma properties along the line of sight (Bromley et al 2001; Broderick & Loeb 2009; Mościbrodzka et al 2017) These polarimetric measurements allow us to carry out new quantitative tests of horizon-scale scenarios for accretion and jet launching around the M87* black hole.
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