Radiative signatures of plasmoid-dominated reconnection in blazar jets

Abstract

The multi-wavelength spectral and temporal variability observed in blazars set tight constraints on current theoretical emission models. Here, we investigate the relativistic magnetic reconnection process as a source of blazar emission in which quasi-spherical plasmoids, containing relativistic particles and magnetic fields, are associated with the emission sites in blazar jets. By coupling recent two-dimensional particle-in-cell simulations of relativistic reconnection with a time-dependent radiative transfer code, we compute the non-thermal emission from a chain of plasmoids formed during a reconnection event. The derived photon spectra display characteristic features observed in both BL Lac sources and flat spectrum radio quasars, with the distinction made by varying the strength of the external photon fields, the jet magnetization, and the number of pairs per proton contained within. Light curves produced from reconnection events are composed of many fast and powerful flares that appear on excess of a slower evolving envelope produced by the cumulative emission of medium-sized plasmoids. The observed variability is highly dependent upon the orientation of the reconnection layer with respect to the blazar jet axis and to the observer. Our model provides a physically motivated framework for explaining the multi-timescale blazar variability across the entire electromagnetic spectrum.

Characterizing the Gamma-Ray Variability of the Brightest Flat Spectrum Radio Quasars Observed with the Fermi LAT

“…No apparent evolution of these quantities with time is observed either. This variety of flare profiles could be explained in the scheme of magnetic reconnection with different orientations of the reconnection layers leading to a variety of Doppler factors of the injected plasmoids (e.g., Petropoulou et al 2016;Christie et al 2019). With our novel approach of identifying flares, it will be possible in the future to build large statistical samples of flares (selected not only by their peak flux but also, e.g., by their integrated flux) whose properties could be compared in more detail to predictions of the reconnection scenario.…”

Unraveling the Complex Behavior of Mrk 421 with Simultaneous X-Ray and VHE Observations during an Extreme Flaring Activity in 2013 April *

“…It has been proposed that plasmoids (i.e., blobs of magnetized plasma containing energetic particles) that are formed and accelerated in the reconnection regions of jets can serve as high-energy emission sites in both blazars and radio galaxies (Giannios et al 2009;Sironi et al 2015). Petropoulou et al 2016, (hereafter, PGS16) presented a semi-analytic model of flares powered by plasmoids in a reconnection layer, simplifying the results of detailed particle-in-cell (PIC) simulations (for a full numerical treatment, see Christie et al 2019).…”

Blazar Optical Polarimetry: Current Progress in Observations and Theories

“…Magnetic reconnection can efficiently dissipate magnetic energy to accelerate particles [49,50,101]. In recent years, it has been well studied, both analytically and numerically, that magnetic reconnection can explain the blazar spectra and light curves [88,[102][103][104][105]. Recently, [106] has combined PIC with polarized radiation transfer simulations to study the polarization signatures during reconnection.…”

“…However, the very dynamical and complex nature of AGN jets prevents the use of simple analytical models. Recent first-principle numerical simulations have attempted to consistently combine plasma dynamics and radiation processes to directly compare with observations [88,98,104,105,107], which have shown interesting predictions in polarization variability. With the development of CPU and GPU computing in the next decade, this multi-physics approach will revolutionize our understanding of jet dynamics.…”