Nonthermal Synchrotron Emission and Polarization Signatures during Black Hole Flux Eruptions

Abstract In this work, we investigate synchrotron emission and the observational signatures of anisotropic nonthermal electrons during magnetic flux eruptions in a magnetically arrested disk, using 3D GRMHD simulations. Nonthermal electrons are assumed to be accelerated 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 nonthermal 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. Strong field-aligned beaming in the electron distribution suppresses nonthermal emission for near-axis observers, effectively driving the image morphology toward a purely thermal limit. In contrast, moderately anisotropic models remain effective at imprinting nonthermal electron signatures on both the total intensity and polarization structure. We further quantify how eruption-driven increases in absorption depth and enhanced Faraday effects reduce the linear polarization fraction and modify the azimuthal coherence of the polarization field. Overall, our results demonstrate that incorporating anisotropic nonthermal electrons is essential for a physically self-consistent interpretation of time-variable Event Horizon Telescope polarimetric observations.