Flares in the Galactic centre – II. Polarization signatures of flares at mm-wavelengths

Abstract

Recent polarimetric mm-observations of the galactic centre showed sinusoidal loops in the $\mathcal {Q{\small --}U}$ 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 are magnetic flux eruptions in magnetically arrested accretion flows (MAD). In our previous work, 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 $\mathcal {Q{\small --}U}$ plane are robustly produced in MAD models as they lead to enhanced emissivity of compressed disc material due to orbiting flux bundles. A timing analysis of the synthetic polarized light curves demonstrate a polarized excess variability at time-scales of $\simeq 1~\rm h$. The polarization loops are also clearly imprinted on the cross-correlation of the Stokes parameters which allows us to extract a typical periodicity of $30~\rm min$ to $1~\rm h$ 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 general relativistic magnetohydrodynamic simulations without saturated magnetic flux, namely, single temperature standard and normal evolution models, also produces $\mathcal {Q{\small --}U}$ loops. However, in disagreement with the observations, loops in these simulations are quasi-continuous with a low polarization excess.