Blazar flares powered by plasmoids in relativistic reconnection

TL;DR

This paper introduces a physically motivated model for blazar flares based on relativistic magnetic reconnection and PIC simulations, explaining rapid variability and spectral features observed in blazar emissions.

Contribution

The study combines PIC simulation results with kinetic modeling to explain blazar flares as a natural outcome of magnetic reconnection in jets, providing a detailed physical mechanism.

Findings

Relativistic reconnection produces plasmoids with high-energy particles and magnetic fields.

Model predicts correlated synchrotron and SSC flares lasting hours to days.

Flares of different durations and luminosities are explained by plasmoid size and speed.

Abstract

Powerful flares from blazars with short ($\sim$ min) variability timescales are challenging for current models of blazar emission. Here, we present a physically motivated ab initio model for blazar flares based on the results of recent particle-in-cell (PIC) simulations of relativistic magnetic reconnection. PIC simulations demonstrate that quasi-spherical plasmoids filled with high-energy particles and magnetic fields are a self-consistent by-product of the reconnection process. By coupling our PIC-based results (i.e., plasmoid growth, acceleration profile, particle and magnetic content) with a kinetic equation for the evolution of the electron distribution function we demonstrate that relativistic reconnection in blazar jets can produce powerful flares whose temporal and spectral properties are consistent with the observations. In particular, our model predicts correlated synchrotron and synchrotron self-Compton flares of duration of several hours--days powered by the largest and slowest moving plasmoids that form in the reconnection layer. Smaller and faster plasmoids produce flares of sub-hour duration with higher peak luminosities than those powered by the largest plasmoids. Yet, the observed fluence in both types of flares is similar. Multiple flares with a range of flux-doubling timescales (minutes to several hours) observed over a longer period of flaring activity (days or longer) may be used as a probe of the reconnection layer's orientation and the jet's magnetization. Our model shows that blazar flares are naturally expected as a result of magnetic reconnection in a magnetically-dominated jet.