Searching for Internal Absorption Signatures in High-redshift Blazars

Abstract The γ -ray emission from flat spectrum radio quasars (FSRQs), a subclass 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 γ -rays through internal photon–photon ( γ – γ ) 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 γ -ray emission region. In this study, we select a sample of γ -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 Large Area Telescope γ -ray spectra of these sources and perform physical modeling using a detailed γ – γ 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 α 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 γ -ray emitting region along the jet. For the remaining, higher-redshift sources, the limited photon statistics prevent a reliable detection of internal opacity features.