Absorption of 10--200 Gev Gamma Rays by Radiation from BLR in Blazars

TL;DR

This study models gamma-ray absorption in blazars due to BLR radiation, predicting that gamma-rays between 10-200 GeV are generally absorbed unless emitted outside the BLR cavity, with implications for the location of gamma-ray emission.

Contribution

The paper introduces a detailed model of gamma-ray absorption considering key BLR parameters, providing predictions for gamma-ray escape based on emission region location.

Findings

Gamma-rays from 10-200 GeV are mostly absorbed if emitted inside the BLR.

Detection of these gamma-rays can indicate emission outside the BLR cavity.

Model predictions can be tested with future GLAST observations.

Abstract

In this paper, we study the photon-photon pair production optical depth for gamma-rays with energies from 10 to 200 GeV emitted by powerful blazars due to the diffuse radiation field of broad line region (BLR). There are four key parameters in the BLR model employed to determine the $\gamma-\gamma$ attenuation optical depth of these gamma-rays. They are the gamma-ray emitting radius $R_{\gamma}$, the BLR luminosity $L_{\rm{BLR}}$, the BLR half thickness $h$ and the ratio $\tau_{\rm{BLR}}/f_{\rm{cov}}$ of the Thomson optical depth to the covering factor of BLR. For FSRQs, on average, it is impossible for gamma-rays with energies from 10 to 200 GeV to escape from the diffuse radiation field of the BLR. If $\it GLAST$ could detect these gamma-rays for most of FSRQs, the gamma-ray emitting region is likely to be outside the cavity formed by the BLR. Otherwise, the emitting region is likely to be inside the BLR cavity. As examples, we estimate the photon-photon absorption optical depth of gamma-rays with energies from 10 to 200 GeV for two powerful blazars, HFSRQ PKS 0405$-$123 and FSRQ 3C 279. Comparing our results with $\it GLAST$ observations in the future could test whether the model employed and the relevant assumptions in this paper are reliable and reasonable, and then limit constraints on the position of the gamma-ray emitting region relative to the BLR and the properties of the BLR.