Gamma-Gamma Absorption in the Broad Line Region Radiation Fields of Gamma-Ray Blazars

TL;DR

This study evaluates gamma-gamma absorption in the Broad Line Region of gamma-ray blazars, showing that gamma-ray emission likely occurs near the BLR's outer boundary to avoid significant absorption.

Contribution

It provides a self-consistent analysis of gamma-gamma absorption considering geometrical effects and spectral energy distribution constraints in FSRQs.

Findings

Optical depth exceeds unity inside the BLR, causing significant absorption.

Gamma-ray emission outside the BLR is implausible due to energy density constraints.

Emission near the outer boundary of the BLR minimizes gamma-gamma absorption.

Abstract

The expected level of gamma-gamma absorption in the Broad Line Region (BLR) radiation field of gamma-ray loud Flat Spectrum Radio Quasars (FSRQs)is evaluated as a function of the location of the gamma-ray emission region. This is done self-consistently with parameters inferred from the shape of the spectral energy distribution (SED) in a single-zone leptonic EC-BLR model scenario. We take into account all geometrical effects both in the calculation of the gamma-gamma opacity and the normalization of the BLR radiation energy density. As specific examples, we study the FSRQs 3C279 and PKS 1510-089, keeping the BLR radiation energy density at the location of the emission region fixed at the values inferred from the SED. We confirm previous findings that the optical depth due to $\gamma\gamma$ absorption in the BLR radiation field exceeds unity for both 3C279 and PKS 1510-089 for locations of the gamma-ray emission region inside the inner boundary of the BLR. It decreases monotonically, with distance from the central engine and drops below unity for locations within the BLR. For locations outside the BLR, the BLR radiation energy density required for the production of GeV gamma-rays rapidly increases beyond observational constraints, thus making the EC-BLR mechanism implausible. Therefore, in order to avoid significant gamma-gamma absorption by the BLR radiation field, the gamma-ray emission region must therefore be located near the outer boundary of the BLR.