Electron-positron cascade in magnetospheres of supermassive Kerr black holes and the origin of relativistic AGN jets

TL;DR

This work analyzes electron–positron cascades in the magnetospheres of rapidly spinning (Kerr) black holes as a condition for launching Blandford–Znajek jets. It derives the cascade’s governing gap physics, including the electric field $E_{\|}$, particle acceleration to a terminal Lorentz factor $\Gamma$, and the thresholds for an avalanche to develop via inverse Compton scattering and photon–photon pair production. The authors show that avalanches operate only for a narrow range of seed-photon energies and accretion-field parameters, and that the resulting lepton supply in luminous AGN jets is several orders of magnitude too small to account for observed jet leptons in sources like 3C 120, unless an additional lepton source is present. Through a detailed case study of 3C 120, they demonstrate that photon-photon pair production from the hot accretion flow can supply the required leptons, while magnetospheric cascades alone fall short, suggesting a combined loading mechanism and offering a potential link to the radio-loud/radio-quiet dichotomy. The results imply that efficient jet production in luminous AGNs likely requires both magnetospheric processes and external lepton sources or baryon loading, with implications for jet composition and energetics.

Abstract

The avalanche mechanism of plasma production in active galactic nuclei (AGNs) is detailed, and constraints on system parameters needed for efficient electron-positron pair cascades are explored. Whether an AGN falls within this favorable parameter range may explain the observed radio-loud versus radio-quiet dichotomy. On the other hand, this study shows that cascades generate orders of magnitude fewer pairs than is necessary to explain the synchrotron emission observed in luminous jets. This fact suggests the existence of either an alternative lepton source, namely pair production of photons from the hot accretion flows around AGN central black holes, or matter loading of the jets from the surrounding medium, or, most likely, both. The case of the radio galaxy 3C 120 is considered in detail.

Figures (1)

• Figure 1: Schematic representation of the accreting BH system. Shown are the ergosphere (green dashed curves), inner and outer light cylinders (gray lines and shades), magnetic field lines (dotted lines), stagnation surface (blue dashed curves), and the GJ null surface (red curves). Yellow-orange and blue indicate positive and negative charge densities, respectively. An accretion disk (not shown) is in the equatorial plane. We assume a rapidly spinning BH with $a_*=0.99$ and $\omega_{\rm F}=0.45\omega_{\rm BH}$. Axes scales are in units of $r_{\rm g}$.