Searching for Internal Absorption Signatures in High-Redshift Blazars

TL;DR

This study investigates internal gamma-ray absorption features in high-redshift blazars to understand their jet structures and emission regions, using Fermi-LAT data and modeling absorption due to the broad line region.

Contribution

It presents the first search for internal absorption signatures in high-redshift FSRQs, constraining the gamma-ray emission region location through spectral analysis.

Findings

Hint of internal absorption in one low-redshift source

Constraints on emission region location for that source

Limited photon statistics prevent detection in higher-redshift sources

Abstract

The gamma-ray emission from Flat Spectrum Radio Quasars (FSRQs), a sub-class of blazars, is believed to be generated through interactions of high-energy leptons and/or hadrons in the jet with the ambient photon fields, including those from the accretion disk, the broad line region (BLR), and the dusty torus. However, these same photon fields can also attenuate gamma-rays through internal photon-photon (gamma-gamma) absorption, imprinting characteristic spectral features. Investigating the internal absorption is crucial for unraveling the complex structure of FSRQs and constraining the poorly known location of the gamma-ray emission region. In this study, we select a sample of gamma-ray detected FSRQs with high redshift (z >= 3), to search for absorption features appearing at lower photon energies due to a substantial redshift. We extract the Fermi-LAT gamma-ray spectra of these sources and perform physical modeling using a detailed gamma-gamma opacity model, assuming that the BLR photon field dominates the absorption and focusing on the energy range ~25 GeV/(1+z), where the absorption feature due to Ly{\alpha} photons is expected. Our analysis reveals a hint of internal absorption for one source (the lowest redshift object in our sample, z~3) and provides constraints on the location of its gamma-ray emitting region along the jet. For the remaining, higher-redshift sources, the limited photon statistics prevent a reliable detection of internal opacity features.