Magnetospheric Gamma-Ray Emission in Active Galactic Nuclei

TL;DR

This paper investigates how magnetospheric models can explain the very high-energy gamma-ray emissions from active galactic nuclei, analyzing the conditions and constraints that influence their observability and variability.

Contribution

It introduces a phenomenological approach to estimate maximum gap power in AGN magnetospheres and assesses the likelihood of magnetospheric origin for observed VHE gamma-ray variability.

Findings

Gap power scales with Blandford-Znajek jet power and gap size.

Radiatively inefficient accretion environments are necessary for VHE transparency.

Day-scale variability in M87 could originate from magnetospheric processes.

Abstract

The rapidly variable, very high-energy (VHE) gamma-ray emission from Active Galactic Nuclei (AGN) has been frequently associated with non-thermal processes occurring in the magnetospheres of their supermassive black holes. The present work aims to explore the adequacy of different gap-type (unscreened electric field) models to account for the observed characteristics. Based on a phenomenological description of the gap potential, we estimate the maximum extractable gap power $L_{gap}$ for different magnetospheric set-ups, and study its dependence on the accretion state of the source. $L_{gap}$ is found to be in general proportional to the Blandford-Znajek jet power $L_{BZ}$ and a sensitive function of gap size $h$, i.e. $L_{gap} \sim L_{BZ} (h/r_g)^{\beta}$, where the power index $\beta \geq 1$ is dependent on the respective gap-setup. The transparency of the black hole vicinity to VHE photons generally requires a radiatively inefficient accretion environment and thereby imposes constraints on possible accretion rates, and correspondingly on $L_{BZ}$. Similarly, rapid variability, if observed, may allow to constrain the gap size $h\sim c \Delta t$. Combining these constraints, we provide a general classification to assess the likelihood that the VHE gamma-ray emission observed from an AGN can be attributed to a magnetospheric origin. When applied to prominent candidate sources these considerations suggest that the variable (day-scale) VHE activity seen in the radio galaxy M87 could be compatible with a magnetospheric origin, while such an origin appears less likely for the (minute-scale) VHE activity in IC310.