From morphology to variability: radiative cooling effects on horizon-scale polarization in two-temperature GRMHD simulations

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ABSTRACT Polarization signatures provide a new window to investigate the effects of radiative cooling in the horizon-scale accretion flows. Morphology and variability of polarization offer quantifiable diagnostics of how cooling modifies the polarized emission from two-temperature GRMHD simulations. We find that cooling enhances the effective Faraday depth, leading to stronger large-scale Faraday scrambling, particularly at higher accretion rates. In contrast, depolarization associated with higher-order photons is comparable between cooling and non-cooling models. Radiative cooling also increases the intrinsic asymmetry in both the ring structure and the polarization pattern. This effect is quantified by enhanced power in non-axisymmetric azimuthal modes ($\beta _m$, $m \ne 2$) relative to the dominant quadrupolar component $\beta _2$. The increased asymmetry is directly linked to stronger temporal variability of the polarization angle $\angle \beta _2$, including frequent sign reversals that are absent in non-cooling models. The radial profile of $\angle \beta _2$ further localizes the physical origin of these effects, distinguishing regions dominated by Faraday rotation from those influenced by photon ring contributions and providing a clear separation between cooling and non-cooling cases. Additional tests including a non-thermal electron population indicate that the polarization structure at 230 GHz is largely insensitive to the detailed form of the electron distribution functions. Our results demonstrate that horizon-scale polarization asymmetry, variability, and radial structure encode robust signatures of radiative cooling. These findings highlight the diagnostic power of time-resolved polarimetry and high-resolution imaging for constraining radiative processes in black hole accretion flows with EHT-like observations.

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