Electron-beam interaction with emission-line clouds in blazars

Abstract

Context: An electron-positron beam escaping from the magnetospheric vacuum gap of an accreting black hole interacts with recombination-line photons from surrounding gas clouds. Inverse-Compton scattering and subsequent pair production initiate unsaturated electromagnetic cascades exhibiting a characteristic spectral energy distribution. Aims: By modelling the interactions of beam electrons (positrons) with hydrogen and helium recombination-line photons, we seek to describe the spectral signature of beam-driven cascades in the broad emission-line region of blazar jets. Methods: Employing coupled kinetic equations for electrons (positrons) and photons including an escape term, we numerically obtain their steady-state distributions, and the escaping photon spectrum. Results: We find that cascade emission resulting from beam interactions can produce a narrow spectral feature at TeV energies. Indications of such an intermittent feature, which defies an explanation in the standard shock-in-jet scenario, have been found at $\approx 4\sigma$ confidence level at an energy of $\approx$ 3 TeV in the spectrum of the blazar Mrk 501. Conclusions: The energetic requirements for explaining the intermittent 3 TeV bump with the beam-interaction model are plausible: Gap discharges that lead to multi-TeV beam electrons (positrons) carrying $\approx$ 0.1 % of the Blandford-Znajek luminosity, which interact with recombination-line photons from gas clouds that reprocess $\approx$ 1 % of the similar accretion luminosity are required.

arXiv commentAccepted for publication in Astronomy and Astrophysics on 07.12.2020. Article comprises 15 pages including the appendices and six figures. Edition with corrected typos and clean grammar

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• Dual-high-frequency VLBI study of blazar-jet brightness-temperature gradients and collimation profiles
• On the narrow spectral feature at $\sim$3 TeV in the MAGIC spectrum of Mrk 501
• Modelling the persistent low-state $\gamma$-ray emission of the PKS 1510-089 blazar with electromagnetic cascades initiated in hadronuclear interactions
• Pair Cascades at the Edge of the Broad-Line Region Shaping the Gamma-Ray Spectrum of 3C 279
• Optically thick, nonlocal, inhomogeneous, stationary jet model for high-energy radiation from blazars: Application to Mrk 421
• TeV gamma-ray spectral spikes produced by magnetic reconnection in blazar jets: the case of the 2014 high state of Markarian 501