Modelling the VHE flare of 3C 279 using one zone leptonic model

TL;DR

This paper models the broadband emission of the quasar 3C 279 during a flare using a one zone leptonic model, highlighting the roles of external Compton scattering and synchrotron self-Compton processes.

Contribution

It demonstrates that external Compton scattering of infrared photons from the dusty torus explains the VHE gamma-ray flux, and distinguishes the emission mechanisms for X-ray and gamma-ray energies.

Findings

EC scattering of dusty torus IR photons explains VHE flux.

SSC process accounts for X-ray emission.

Model reproduces broadband spectrum but predicts high MeV-GeV flux.

Abstract

We model the simultaneous observations of the flat spectrum radio quasar 3C 279 at radio, optical, X-ray and very high energy (VHE) gamma ray energies during 2006 flare using a simple one zone leptonic model. We consider synchrotron emission due to cooling of a non-thermal electron distribution in an equipartition magnetic field and inverse Compton emission due to the scattering off synchrotron photons (SSC) and external soft photons (EC) by the same distribution of electrons. We show that the VHE gamma ray flux cannot be explained by SSC process thereby suggesting the EC mechanism as a plausible emission process at this energy. The EC scattering of BLR photons to VHE energies will be in Klein-Nishina regime predicting a steep spectrum which is contrary to the observations. However the infrared photons from the dusty torus can be boosted to VHE energies with the scattering process remaining in the Thomson regime. Though EC process can successfully explain the observed VHE flux, it require a magnetic field much lower than the equipartition value to reproduce the observed X-ray flux. Hence we attribute the X-ray emission due to SSC process. We derive the physical parameters of 3C 279 considering the above mentioned emission processes. In addition we assume the size of emission region constrained by a variability timescale of one day. This model can successfully reproduce the broadband spectrum of 3C 279 but predicts substantially large flux at MeV-GeV energies.