[Paper Review] Flares in the Galactic Centre II: polarisation signatures of flares at mm-wavelengths

Mahdi Najafi-Ziyazi, Jordy Davelaar+2 more|arXiv (Cornell University)|Aug 31, 2023

Solar and Space Plasma DynamicsPhysics and Astronomy3 citations

TL;DR

This study investigates polarized mm-wavelength flares in the Galactic Centre using GRMHD simulations, demonstrating that magnetic flux eruptions in magnetically arrested disk (MAD) models naturally produce Q-U polarization loops resembling those observed by ALMA. The loops arise from orbiting, compressed flux bundles enhancing local emissivity, with synthetic light curves showing ~1 hr periodicity and robust polarization variability, supporting MAD models as a viable explanation for observed flares.

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 are 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

Motivation & Objective

• To determine whether magnetic flux eruptions in magnetically arrested accretion flows (MAD) can produce the observed Q-U polarization loops in the Galactic Centre's mm-wavelength flares.
• To test if the timing and morphology of polarization loops in ALMA observations (Wielgus et al., 2022a) can be reproduced by GRMHD simulations of MAD and SANE models.
• To assess the robustness of polarization loop features across different viewing angles and black hole spin configurations.
• To compare synthetic polarized light curves with observations, particularly focusing on periodicity and polarization excess at ~1 hr timescales.
• To evaluate the role of optical depth and electron temperature in shaping spectral indices and flux variability in mm-wavelength flares.

Proposed method

• Simulate magnetically arrested disk (MAD) and single-temperature SANE GRMHD models using general relativistic magnetohydrodynamics with a realistic electron temperature prescription.
• Compute Stokes parameters (I, Q, U) from thermal synchrotron emission using the GRMHD simulation outputs, accounting for relativistic effects and radiation transfer.
• Generate synthetic polarized light curves and analyze their temporal variability, focusing on periodic features at ~30 min to 1 hr timescales.
• Construct cross-correlations of Stokes Q and U to extract characteristic periodicity and assess its dependence on black hole spin and viewing angle.
• Compare simulated Q-U trajectories with observed ALMA loops, including directionality (clockwise vs. counter-clockwise) and loop morphology.
• Assess spectral index evolution (α) at 230 GHz and evaluate consistency with observed values (α ≥ 0 in quiescence, ≈ -0.2 post-flare).

Experimental results

Research questions

• RQ1 Can magnetic flux eruptions in MAD models produce Q-U polarization loops that match the ALMA-observed sinusoidal loops in the Galactic Centre?
• RQ2 What is the characteristic timescale of polarization variability in synthetic light curves from GRMHD simulations, and does it align with the observed ~1 hr periodicity?
• RQ3 How does the polarization loop morphology and periodicity depend on black hole spin and viewing angle in MAD models?
• RQ4 Do SANE models also produce similar Q-U loops, and if so, how do their properties differ from those in MAD models?
• RQ5 To what extent do simulated spectral indices at 230 GHz match the observed values, and what role does optical depth play in this discrepancy?

Key findings

• Robust Q-U polarization loops are produced in MAD models due to orbiting, compressed magnetic flux bundles enhancing local emissivity, matching the observed ALMA loop morphology.
• Synthetic polarized light curves exhibit a clear excess variability at timescales of approximately 1 hour, consistent with the observed flare periodicity.
• Cross-correlation of Stokes Q and U reveals a characteristic periodicity of 30–60 minutes, with some evidence for spin dependence, supporting the intrinsic nature of the signal in MAD states.
• The polarization loops are robustly reproduced across different viewing angles, though clockwise rotation requires an inclination >90°, which is consistent with observational geometry.
• SANE models also produce Q-U loops, but they are quasi-continuous and exhibit low polarization excess, differing from the sharp, high-contrast loops observed in ALMA data.
• The simulated spectral indices (α ∈ [-1, -0.2]) are too negative compared to observations (α ≥ 0 in quiescence), suggesting models may be too optically thin; colder electron temperatures are needed to increase optical depth and spectral index.