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

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.

arXiv comment10 pages, 3 figures, 1 table, Published in ApJL

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