Constraining the $\gamma$-ray Emission Region for Fermi-Detected FSRQs by the Seed Photon Approach

TL;DR

This paper uses a seed photon factor approach with spectral energy distributions of FSRQs to locate the gamma-ray emission region, concluding it occurs near the dusty torus far from the black hole, supporting a far-site emission scenario.

Contribution

It introduces a seed photon factor method to constrain the gamma-ray emission region in FSRQs, providing evidence for a far-site origin near the dusty torus.

Findings

Gamma-ray emission occurs beyond the broad-line region.

The emission region is close to the dusty torus at parsec scales.

Supports the far-site scenario for gamma-ray production in blazars.

Abstract

The location of $\gamma$-ray emitting region in blazars has been an open issue for several decades and is still being debated. We use the Paliya et al. sample of 619 $\gamma$-ray-loud flat-spectrum radio quasars with the available spectral energy distributions, and employ a seed photon factor approach, to locate the $\gamma$-rays production region. This method efficiently set up a relation between the peak frequencies and luminosities for the synchrotron emission and inverse Compton scattering, together with a combination of the energy density and characteristic energy for the external seed photon field, namely, $\sqrt{U_0}/\epsilon_0$, an indicative factor of seed photons (SF) in units of Gauss. By means of comparing it with canonical values of broad-line region and molecular dusty torus, we principally ascertain that the GeV emission is originated far beyond the BLR and close to the DT -- farther out at pc scales from the central black hole, which supports a {\it far-site} scenario for $\gamma$-ray blazars. We probe the idea that inverse Compton scattering of infrared seed photons is happening in the Thomson regime. This approach and our findings are based on the validity of the External Compton model, which is applicable to understand the GeV emission mechanism in FSRQs. However, the completeness of this framework has been challenged by reports of neutrino emission from blazars. Thus we also shed new light on the neutrino production region by using our derived results since blazars are promising neutrino emitters.