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.

10 pages, 3 figures, 1 table, Published in ApJL

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Cited by in corpus (3)

• Insights from leptohadronic modelling of the brightest blazar flare
• Constraining $\gamma$-ray dissipation site in gravitationally lensed quasar -- PKS 1830$-$211
• Pinpointing the location of the gamma-ray emitting region in the FSRQ 4C+01.28