The feasibility of magnetic reconnection powered blazar flares from synchrotron self-Compton emission

Abstract

Order of magnitude variability has been observed in the blazar sub-class of Active Galactic Nuclei on minute timescales. These high-energy flares are often difficult to explain with shock acceleration models due to the small size of the inferred emitting region, with recent particle-in-cell (PIC) simulations showing that magnetic reconnection is a promising alternative mechanism. Here, we present a macroscopic emission model physically motivated by PIC simulations, where the energy for particle acceleration originates from the reconnecting magnetic field. We track the radial growth and relative velocity of a reconnecting plasmoid, modelling particle acceleration and radiative losses from synchrotron and synchrotron self-Compton (SSC) emission. To test the viability of magnetic reconnection as the mechanism behind rapid blazar flares we simultaneously fit our model to the observed light-curve and SED from the 2016 TeV flare of BL Lacertae. We find generally that, without considering external photons, reconnecting plasmoids are unable to produce Compton-dominant TeV flares and so cannot reproduce the observations due to overproduction of synchrotron emission. Additionally, problematically large plasmoids, comparable in size to the entire jet radius, are required to emit sufficient SSC gamma-rays to be observable. However, our plasmoid model can reproduce the rapid TeV lightcurve of the flare, demonstrating that reconnection is able to produce rapid, powerful TeV flares on observed timescales. We conclude that while reconnection can produce SSC flares on the correct timescales, the primary source of TeV emission cannot be SSC and the size of plasmoids required may be implausibly large.

Interplasmoid Compton scattering and the Compton dominance of BL Lacs

“…When adopting single-zone emission models under the assumption of equipartition, as is expected for reconnection driven blazar flares, BL Lac-like models are found to have Compton ratios of A C 0.1. More recently, the BL Lac plasmoid-emission models of PGS16, C19, and Morris et al (2019), showed that the average A C is ∼ 0.2, which falls near the lower bound of what is typically observed.…”

Radiation and Polarization Signatures from Magnetic Reconnection in Relativistic Jets. I. A Systematic Study

“…This region typically occupies only a few to ten percent of the merging plasmoids. Following the calculation in Morris et al (2019) for instance, the synchrotron photon energy density during plasmoid mergers will be higher than their estimates by at least one order, due to the highly inhomogeneous synchrotron emission. Since the SSC luminosity is proportional to the synchrotron photon energy density, this means that the Compton dominance is comparable to or larger than 1.…”

“…It is important to note that previous studies have found the Compton dominance, which is the ratio between the luminosity of the Compton scattering component over the synchrotron component, is typically less than 0.1 (Morris et al 2019;Christie et al 2019, however, see Christie et al 2020, which suggests that local Doppler boosting can enhance the Compton dominance). We note that these previous works generally assumed that the synchrotron photons are distributed over the entire plasmoid.…”

Broadband study of BL Lac during flare of 2020: Spectral evolution and emergence of HBL component

“…Further, the lepto-hadronic modeling was also used to modeled the SED of BL Lac where the high magnetic field and high proton power is required. A fast flare in BL Lac was also observed in very high energy (VHE) γ-ray by MAGIC and VERITAS in 2015 and 2016 and modeled by IC (MAGIC Collaboration et al 2020) and SSC mechanism (Morris et al 2019). For the comparison purpose, we have modeled the broadband SED of two instances as marked in Figure 7 by vertical pink dashed line.…”