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Magnetospheric Gamma-Ray Emission in Active Galactic Nuclei

The rapidly variable, very high-energy (VHE) gamma-ray emission from active galactic nuclei (AGNs) 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 Lgap for different magnetospheric setups, and study its dependence on the accretion state of the source. Lgap is found in general to be proportional to the Blandford–Znajek jet power LBZ and a sensitive function of gap size h, i.e., , where the power index is dependent on the respective gap setup. The transparency of the vicinity of the black hole to VHE photons generally requires a radiatively inefficient accretion environment and thereby imposes constraints on possible accretion rates, and correspondingly on LBZ. Similarly, rapid variability, if observed, may allow one to constrain the gap size . 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 IC 310.