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 obtained 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 ≈ 4σ confidence level at an energy of ≈ 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 ≈ 0.1% of the Blandford-Znajek luminosity, which interact with recombination-line photons from gas clouds that reprocess ≈ 1% of the similar accretion luminosity are required.