Imprint of "Local Opacity" Effect in Gamma-Ray Spectrum of Blazar Jet

TL;DR

This paper presents evidence of an intrinsic gamma-ray absorption feature in a blazar, indicating the gamma-ray emission region is near the outer edge of the broad-line region, with implications for jet dissipation mechanisms.

Contribution

It identifies a specific absorption feature in gamma-ray spectra linked to the BLR, providing new insights into the location and conditions of gamma-ray emission in blazar jets.

Findings

Detection of an absorption feature above 10 GeV in gamma-ray spectrum.

Absence of the feature during low-flux states supports the BLR-related origin.

Emission zone location consistent with variability timescales and external Compton scattering.

Abstract

Relativistic jets from accreting supermassive black holes at cosmological distances can be powerful emitters of $\gamma$-rays. However, the precise mechanisms and locations responsible for the dissipation of energy within these jets, leading to observable $\gamma$-ray radiation, remain elusive. We detect evidence for an intrinsic absorption feature in the $\gamma$-ray spectrum at energies exceeding $10\,$GeV, presumably due to the photon-photon pair production of $\gamma$-rays with low ionization lines at the outer edge of Broad-line region (BLR), during the high-flux state of the flat-spectrum radio quasar PKS 1424$-$418. The feature can be discriminated from the turnover at higher energies resulting from $\gamma$-ray absorption in the extragalactic background light. It is absent in the low-flux states supporting the interpretation that powerful dissipation events within or at the edge of the BLR evolve into fainter $\gamma$-ray emitting zones outside the BLR, possibly associated with the moving VLBI radio knots. The inferred location of $\gamma$-ray emission zone is consistent with the observed variability time scale of the brightest flare, provided that the flare is attributed to external Compton scattering with BLR photons.