A fully-kinetic model for orphan gamma-ray flares in blazars

TL;DR

This paper introduces a fully-kinetic model explaining orphan gamma-ray flares in blazars as a result of particle energization in turbulent magnetically-dominated plasmas, emphasizing the role of particle anisotropy.

Contribution

The study presents the first self-consistent kinetic simulation model for orphan gamma-ray flares, highlighting the importance of particle anisotropy and turbulence in blazar emission.

Findings

Orphan gamma-ray flares can result from particle energization in turbulent plasmas.

Particle anisotropy suppresses synchrotron emission while enabling gamma-ray flares.

Turbulence level influences the ratio of inverse Compton to synchrotron luminosity.

Abstract

Blazars emit a highly-variable non-thermal spectrum. It is usually assumed that the same non-thermal electrons are responsible for the IR-optical-UV emission (via synchrotron) and the gamma-ray emission (via inverse Compton). Hence, the light curves in the two bands should be correlated. Orphan gamma-ray flares (i.e., lacking a luminous low-frequency counterpart) challenge our theoretical understanding of blazars. By means of large-scale two-dimensional radiative particle-in-cell simulations, we show that orphan gamma-ray flares may be a self-consistent by-product of particle energization in turbulent magnetically-dominated pair plasmas. The energized particles produce the gamma-ray flare by inverse Compton scattering an external radiation field, while the synchrotron luminosity is heavily suppressed since the particles are accelerated nearly along the direction of the local magnetic field. The ratio of inverse Compton to synchrotron luminosity is sensitive to the initial strength of turbulent fluctuations (a larger degree of turbulent fluctuations weakens the anisotropy of the energized particles, thus increasing the synchrotron luminosity). Our results show that the anisotropy of the non-thermal particle population is key to modeling the blazar emission.