Variability and Spectral Behavior of Gamma-ray Flares of 3C 279

TL;DR

This study analyzes gamma-ray flares of 3C 279 in 2018, revealing spectral behaviors, modeling the emission with external inverse Compton scattering of dust torus photons, and suggesting external electron injection as the flare driver.

Contribution

It provides detailed spectral analysis and modeling of 3C 279's gamma-ray flares, proposing a new scenario involving external electron injection and ruling out shock acceleration and magnetic reconnection.

Findings

Spectra show no clear breaks, indicating dissipation outside the broad-line region.

Flare decay timescales match electron cooling times with dust torus photons.

Electron energy distribution is harder, implying external electron injection.

Abstract

3C 279 showed enhanced flux variations in Fermi-LAT {\gamma}-ray observations from January to June 2018. We present a detailed Fermi-LAT analysis to investigate the variability and spectral behaviors of 3C 279 during the {\gamma}-ray flares in 2018. In this work, we analyzed the {\gamma}-ray spectra and found that the spectra in either the flaring or quiescent states do not show any clear breaks (or cutoffs). This indicates that the dissipation region is outside the broad-line region, and the energy dissipation may be due to the inverse Compton process of scattering the dust torus infrared photons, this result is also consistent with that in Tolamatti et al. An external inverse Compton scattering of dusty torus (DT) photons is employed to calculate the broadband spectral energy distribution (SED). This model was further supported by the fact that we found flare decay timescale was consistent with the cooling time of relativistic electrons through DT photons. During the SED modeling, a relatively harder spectrum for the electron energy distribution (EED) is found and suggests these electrons may not be accelerated by the shock that happened in the dissipation region. Besides, the magnetic reconnection is also ruled out due to a low magnetization ratio. Thus, we suggest an injection of higher-energy electrons from outside the blob and raising the flare.