Non-thermal Synchrotron Emission and Polarization Signatures during Black Hole Flux Eruptions

Abstract

In this work, we investigate synchrotron emission and the observational signatures of anisotropic non-thermal electrons during magnetic-flux eruptions in a magnetically arrested disk, using 3D GRMHD simulations. Non-thermal electrons are assumed to be energized from the thermal background through magnetic reconnection, with pitch-angle distributions modeled as beamed or loss-cone types, alongside an isotropic case for comparison. The results show that non-thermal emission can produce pronounced flux outbursts and localized brightening during eruptions, while the associated increase in optical depth can suppress the linear polarization fraction. Introducing pitch-angle anisotropy further reshapes the angular distribution of the intrinsic emissivity and modulates its contribution to various observable signatures. Our results demonstrate that anisotropic non-thermal electrons are essential for a physically complete interpretation of black hole image variability.

Figures (15)

• Figure 1: Time evolutions of accretion rate, magnetic flux, and the MAD parameter. The horizontal black line marks $\phi_\text{EH} = 15$. We identify flux-eruption events as the pink bands, where $\Phi_\text{EH}$ drops steeply from a local maximum to a subsequent local minimum.
• Figure 2: Density profiles in the $x-z$ plane (top) and $x-y$ plane (bottom) at $t = 11210 \,t_\text{g}$, $t = 11330 \,t_\text{g}$ and $t = 11460 \,t_\text{g}$. The dark green solid contour represents $-hu_t = 1.05$, the dark green dashed contour indicates the magnetization $\sigma_\text{M} = 20$, and the black arrows depict the magnetic field lines (the same below).
• Figure 3: Angular dependence of synchrotron emissivities for eDFs with with (left) and without (right) the $Z_2$ symmetry, shown as functions of $\alpha$ within the fluid comoving frame. The axis $\alpha = 0^{\circ}$ denotes the direction of the local magnetic field $b$.
• Figure 4: Time evolution of 230 GHz luminous flux for different eDF models. Pink bands indicate the third magnetic flux eruption event.
• Figure 5: Intensity maps overlaid with linear polarizations at 230 GHz from synchrotron emission of the thermal model ${\mathcal{T}}$ (top) and hybrid model ${\mathcal{P}}$ (down), evaluated at four time instances: $t=10800 \,t_\text{g}$, $11210 \,t_\text{g}$, $11330 \,t_\text{g}$, and $11460 \,t_\text{g}$ (columns from left to right). The unit of the intensity is $\text{erg} \,\,\text{s}^{-1} \text{cm}^{-2} \text{sr}^{-1} \text{Hz}^{-1}$.