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ABSTRACT Magnetic reconnection is often considered as the primary particle acceleration mechanism in a magnetized blazar zone environment. The majority of radiation in the reconnection layer comes from plasmoids and their mergers. In particular, plasmoid mergers can produce strong multiwavelength flares and major variations in synchrotron polarization signatures. However, radiative properties of plasmoid mergers have not been well explored due to difficulties in tracking the merging processes. Here we use an image processing method that combines the magnetic vector potential and density to identify isolated and merging plasmoids. We find that this method can clearly distinguish radiation contributions from isolated plasmoids, merging plasmoids, and the primary current sheet of reconnection. This new method enables us to study the radiative properties of plasmoids and mergers statistically. Our results show that isolated plasmoids have similar emissivity regardless of their sizes, and they generally have non-zero polarization degree (PD) due to their quasi-circular shape. Flares due to plasmoid mergers have relative amplitudes that are antiproportional to the size ratio of the plasmoids participating in the mergers. Finally, only mergers between plasmoids of comparable sizes (width ratio ≲5) can lead to significant spectral hardening and polarization angle (PA) variations; the amplitude of the PA variations is between 0 and 180° and has a mean value of 90°. Our analyses on 2D simulations can pave the way for future analyses and machine learning techniques on radiative properties of 3D magnetic reconnection simulations. 

ABSTRACT Relativistic jets are highly collimated plasma outflows emerging from accreting black holes. They are launched with a significant amount of magnetic energy, which can be dissipated to accelerate non-thermal particles and give rise to electromagnetic radiation at larger scales. Kink instabilities can be an efficient mechanism to trigger dissipation of jet magnetic energy. While previous works have studied the conditions required for the growth of kink instabilities in relativistic jets, the radiation signatures of these instabilities have not been investigated in detail. In this paper, we aim to self-consistently study radiation and polarization signatures from kink instabilities in relativistic jets. We combine large-scale relativistic magnetohydrodynamic (RMHD) simulations with polarized radiation transfer of a magnetized jet, which emerges from the central engine and propagates through the surrounding medium. We observe that a localized region at the central spine of the jet exhibits the strongest kink instabilities, which we identify as the jet emission region. Very interestingly, we find quasi-periodic oscillation (QPO) signatures in the light curve from the emission region. Additionally, the polarization degree appears to be anticorrelated to flares in the light curves. Our analyses show that these QPO signatures are intrinsically driven by kink instabilities, where the period of the QPOs is associated with the kink growth time-scale. The latter corresponds to weeks to months QPOs in blazars. The polarization signatures offer unique diagnostics for QPOs driven by kink instabilities.