Flares in the Galactic centre II: Polarisation signatures of flares at mm-wavelengths
Abstract
Recent polarimetric mm-observations of the galactic centre showed sinusoidal loops in the $\mathcal {Q-U}$ 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 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 $\mathcal {Q-U}$ 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 $\simeq 1~\rm hr$. The polarization loops are also clearly imprinted on the cross-correlation of the Stokes parameters which allows us to extract a typical periodicity of $30~\rm min$ to $1~\rm hr$ 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, namely, single temperature standard and normal evolution (SANE) models, also produces $\mathcal {Q-U}$ loops. However, in disagreement with the observations, loops in these simulations are quasi-continuous with a low polarization excess.
- Publication:
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Monthly Notices of the Royal Astronomical Society
- Pub Date:
- May 2024
- DOI:
- 10.1093/mnras/stae1343
- arXiv:
- arXiv:2308.16740
- Bibcode:
- 2024MNRAS.tmp.1347N
- Keywords:
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- Astrophysics - High Energy Astrophysical Phenomena;
- General Relativity and Quantum Cosmology
- E-Print:
- submitted to MNRAS