A diagnostic test for determining the location of the GeV emission in powerful blazars

TL;DR

This paper introduces a diagnostic method to determine whether GeV emission in powerful blazars originates within the broad emission line region or further out in the molecular torus, based on variability patterns.

Contribution

It provides a new diagnostic tool using variability patterns to locate the GeV emission site in blazars, distinguishing between BLR and MT regions.

Findings

Energy-independent variability indicates emission within the BLR.

Energy-dependent variability suggests emission outside the BLR.

Numerical simulations support the diagnostic method.

Abstract

An issue currently under debate in the literature is how far from the black hole is the Fermi-observed GeV emission of powerful blazars emitted. Here we present a clear diagnostic tool for testing whether the GeV emission site is located within the sub-pc broad emission line (BLR) region or further out in the few pc scale molecular torus (MT) environment. Within the BLR the scattering takes place at the onset of the Klein-Nishina regime, causing the electron cooling time to become almost energy independent and as a result, the variation of high-energy emission is expected to be achromatic. Contrarily, if the emission site is located outside the BLR, the expected GeV variability is energy-dependent and with amplitude increasing with energy. We demonstrate this using time-dependent numerical simulations of blazar variability.