Simulations of polarization signatures in the Galactic centre

Companion movies to the paper "Flares in the Galactic centre II: polarisation signatures of flares at mm-wavelengths". GRMHD simulations performed with BHAC (www.bhac.science), GR-ratracing with RAPTOR (https://github.com/jordydavelaar/raptor). Abstract: Recent polarimetric mm-observations of the galactic centre (Sagittarius A*, Sgr A*) 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. Recent advancements in understanding the accretion processes of Sgr A* by the Event Horizon Telescope suggest that the flow is best described with a magnetically arrested disk (MAD) model. MAD models self-consistently produce periodic magnetic flux eruptions as the total magnetic flux on the black hole’s event horizon saturates. This offers a dynamical explanation to the frequent flaring of the Galactic center. In our previous work (Porth et al. 2021), we studied the accretion flow dynamics during these flux eruptions, while omitting the emission. In this work, we extend our previous study by investigating whether the polarization loops observed in Sgr A* can be a signature produced by magnetic flux eruptions. We study a library of general relativistic magnetohydrodynamic (GRMHD) models for varying black hole spin and post-process these simulations with a polarized radiation transfer code to generate 230 GHz light curves of the full Stokes vector for varying plasma conditions, e.g. disk temperatures, as well as inclination angles. We find that loops in the Q − U plane are robustly produced in MAD models during magnetic flux eruptions as they lead to enhanced emissivity of compressed disk material due to orbiting flux bundles. We also perform a timing analysis of the polarized lightcurves which 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 us 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 thus should 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; furthermore these models are disfavored by the EHT modeling