Imprint of 'local opacity' effect in gamma-ray spectrum of blazar jet

Relativistic jets from accreting supermassive black holes at cosmological distances can be powerful emitters of γ-rays. However, the precise mechanisms and locations responsible for the dissipation of energy within these jets, leading to observable γ-ray radiation, remain elusive. We detect evidence for an intrinsic absorption feature in the γ-ray spectrum at energies exceeding 10GeV, presumably due to the photon-photon pair production of γ-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 γ-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 γ-ray emitting zones outside the BLR, possibly associated with the moving VLBI radio knots. The inferred location of γ-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.