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 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.

Gamma-ray signatures from pair cascades in recombination-line radiation fields

“…For given 0 , i and and after choosing an initial , it can be solved iteratively for the electron distribution . Discretising into a grid of 140 points, beginning at i, 0 and iterating towards lower energies, we achieve approximate convergence after a maximum number of ≈ 100 iteration steps [8]. At TeV energies, is shaped mainly due to the injected electrons.…”

“…To describe the linear cascade, we specify all energy loss and gain rates of the electrons and of the gamma-ray photons, respectively [8,16]. After assuming steady state, we arrive at an equation ( ) = F ( 0 , i ( ), , ( ), ).…”

“…According to our model sketched in Fig. 2, the narrow feature at ∼ 3 TeV can be a signature of vacuum gap activity in the magnetosphere of Mrk 501 [6][7][8]. Around a rotating black hole (BH) the co-rotating magnetic field induces an electric field.…”

“…Gamma-ray signatures from pair cascades Christoph Wendel [8], which has a peak at ≈ mean due to IC up-scattering of the soft photons by the injected electrons. Below this peak, the cascaded spectrum shows absorption troughs due to the maxima of the pair production cross section contributions of the various lines.…”

VHE gamma-ray spectral hint of two-zone emitting region in Mrk 501

“…The results on the data analysis in a MWL context are published in [1]. The theoretical framework where pair cascade model is described in detail can be found in [2]. More details on the development of the pair cascade model can be found in the Talk#141.…”