Flaring activity from magnetic reconnection in BL Lacertae

TL;DR

This study analyzes the spectral energy distribution and rapid variability of BL Lac during a major flare, proposing magnetic reconnection and mini-jet models to explain observed phenomena and constraining jet magnetic fields and reconnection regions.

Contribution

It introduces a magnetic reconnection-based mini-jet model to explain rapid variability and spectral shifts in blazar flares, challenging shock acceleration explanations.

Findings

Observed minute-scale gamma-ray flares with spectral shifts.

Magnetic reconnection models explain rapid variability without high Doppler factors.

Magnetic field in reconnection region estimated at ~0.6 G.

Abstract

The evolution of the spectral energy distribution during flares constrains models of particle acceleration in blazar jets. The archetypical blazar BL Lac provided a unique opportunity to study spectral variations during an extended strong flaring episode from 2020-2021. During its brightest $\gamma$-ray state, the observed flux (0.1-300 GeV) reached up to $2.15\,\times\,10^{-5}\,\rm{ph\,cm^{-2}\,s^{-1}}$, with sub-hour scale variability. The synchrotron hump extended into the X-ray regime showing a minute-scale flare with an associated peak shift of inverse-Compton hump in gamma-rays. In shock acceleration models, a high Doppler factor value $>$100 is required to explain the observed rapid variability, change of state, and $\gamma$-ray peak shift. Assuming particle acceleration in mini-jets produced by magnetic reconnection during flares, on the other hand, alleviates the constraint on required bulk Doppler factor. In such jet-in-jet models, observed spectral shift to higher energies (towards TeV regime) and simultaneous rapid variability arises from the accidental alignment of a magnetic plasmoid with the direction of the line of sight. We infer a magnetic field of $\sim0.6\,\rm{G}$ in a reconnection region located at the edge of BLR ($\sim0.02\,\rm{pc}$). The scenario is further supported by log-normal flux distribution arising from merging of plasmoids in reconnection region.