Publications
Authors: The Event Horizon Telescope Collaboration
Abstract: In a companion paper, we present the first spatially resolved polarized image of Sagittarius A∗ on event horizon scales, captured using the Event Horizon Telescope, a global very long baseline interferometric array operating at a wavelength of 1.3 mm. Here, we interpret this image using both simple analytic models and numerical general relativistic magnetohydrodynamic (GRMHD) simulations. The large spatially resolved linear polarization fraction (24-28%, peaking at ∼40%) is the most stringent constraint on parameter space, disfavoring models that are too Faraday depolarized. Similar to our studies of M87∗ , polarimetric constraints reinforce a preference for GRMHD models with dynamically important magnetic fields. Although the spiral morphology of the polarization pattern is known to constrain the spin and inclination angle, the time-variable rotation measure (RM) of Sgr A∗ (equivalent to ≈46◦ ± 12◦ rotation at 228 GHz) limits its present utility as a constraint. If we attribute the RM to internal Faraday rotation, then the motion of accreting material is inferred to be counter-clockwise, contrary to inferences based on historical polarized flares, and no model satisfies all polarimetric and total intensity constraints. On the other hand, if we attribute the mean RM to an external Faraday screen, then the motion of accreting material is inferred to be clockwise, and one model passes all applied total intensity and polarimetric constraints: a model with strong magnetic fields, a spin parameter of 0.94, and an inclination of 150◦. We discuss how future 345 GHz and dynamical imaging will mitigate our present uncertainties and provide additional constraints on the black hole and its accretion flow.
Published in Astrophysical Journal on March 2024
Authors: Mahdi Najafi-Ziyazi, Jordy Davelaar, Yosuke Mizuno, Oliver Porth
Abstract: Recent polarimetric mm-observations of the galactic centre by Wielgus et al. (2022a) showed sinusoidal loops in the Q − U plane with a duration of one hour. The loops coincide with a quasi-simultaneous X-ray flare. A promising mechanism to explain the flaring events is magnetic flux eruptions in magnetically arrested accretion flows (MAD). In our previous work (Porth et al. 2021), we studied the accretion flow dynamics during flux eruptions. Here, we extend our previous study by investigating whether polarization loops can be a signature produced by magnetic flux eruptions. We find that loops in the Q − U plane are robustly produced in MAD models as they lead to enhanced emissivity of compressed disk material due to orbiting flux bundles. A timing analysis of the synthetic polarized lightcurves demonstrate a polarized excess variability at timescales of ≃ 1 hr. The polarization loops are also clearly imprinted on the cross-correlation of the Stokes parameters which allows to extract a typical periodicity of 30 min to 1 hr with some evidence for a spin dependence. These results are intrinsic to the MAD state and should thus hold for a wide range of astrophysical objects. A subset of GRMHD simulations without saturated magnetic flux (single temperature SANE models) also produces Q − U loops. However, in disagreement with the findings of Wielgus et al. (2022a), loops in these simulations are quasi-continuous with a low polarization excess.
Published in MNRAS on May 2024