A method for localizing energy dissipation in blazars using Fermi variability

TL;DR

This paper introduces a method to determine whether gamma-ray emission in blazars occurs within the broad line region or farther out in the molecular torus by analyzing energy-dependent variability patterns.

Contribution

The paper presents a novel approach to localize energy dissipation sites in blazars using gamma-ray variability characteristics related to inverse Compton scattering regimes.

Findings

Energy-independent gamma-ray variability indicates emission within the BLR.

Energy-dependent variability suggests emission occurs in the molecular torus.

The method can distinguish emission regions based on electron cooling time patterns.

Abstract

The distance of the Fermi-detected blazar gamma-ray emission site from the supermassive black hole is a matter of active debate. Here we present a method for testing if the GeV emission of powerful blazars is produced within the sub-pc scale broad line region (BLR) or farther out in the pc-scale molecular torus (MT) environment. If the GeV emission takes place within the BLR, the inverse Compton (IC) scattering of the BLR ultraviolet (UV) seed photons that produces the gamma-rays takes place at the onset of the Klein-Nishina regime. This causes the electron cooling time to become practically energy independent and the variation of the gamma-ray emission to be almost achromatic. If on the other hand the gamma-ray emission is produced farther out in the pc-scale MT, the IC scattering of the infrared (IR) MT seed photons that produces the gamma-rays takes place in the Thomson regime, resulting to energy-dependent electron cooling times, manifested as faster cooling times for higher Fermi energies. We demonstrate these characteristics and discuss the applicability and limitations of our method.