Issue |
A&A
Volume 646, February 2021
|
|
---|---|---|
Article Number | A115 | |
Number of page(s) | 14 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361/202038343 | |
Published online | 17 February 2021 |
Electron-beam interaction with emission-line clouds in blazars
1
Universität Würzburg, 97074 Würzburg, Germany
e-mail: cwendel@astro.uni-wuerzburg.de
2
Inst. de Astrofísica de Canarias, 38200 La Laguna, Spain
e-mail: jbecerra@iac.es
3
Universidad de La Laguna, Dpto. Astrofísica, 38206 La Laguna, Tenerife, Spain
4
Max-Planck-Institut für Physik, 80805 München, Germany
e-mail: dpaneque@mppmu.mpg.de
Received:
4
May
2020
Accepted:
7
December
2020
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.
Key words: galaxies: active / BL Lacertae objects: individual: Markarian 501 / quasars: emission lines / acceleration of particles / scattering / radiation mechanisms: non-thermal
© ESO 2021
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