Electron-beam interaction with emission-line clouds in blazars

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 charac...

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Bibliographic Details
Published in:Astronomy and astrophysics (Berlin) 2021-02, Vol.646, p.A115
Main Authors: Wendel, Christoph, Becerra González, Josefa, Paneque, David, Mannheim, Karl
Format: Article
Language:English
Subjects:
Beam interactions
Blazars
Cascades
Clouds
Confidence intervals
Deposition
Elastic scattering
Electron beams
Electron recombination
Emission
Interaction models
Kinetic equations
Luminosity
Magnetospheres
Pair production
Photons
Positron beams
Positrons
Spectra
Spectral energy distribution
Spectral signatures
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Summary: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 ≈ 4 σ 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.
ISSN:0004-6361
1432-0746
DOI:10.1051/0004-6361/202038343